Reversible bifunctional air electrode catalyst for rechargeable metal air battery and regenerative fuel cell

ABSTRACT

An electrochemical cell includes an air electrode in flow communication with a storage tank containing an aqueous solution of hydrogen peroxide, a lithium electrode, a catalyst layer in contact with the air electrode or a gas diffusion layer associated with the air electrode, and a separator layer in contact with the lithium electrode and catalyst layer. The catalyst layer includes a catalyst for two electron reversible oxygen reduction. The catalyst comprises gold, and a cobalt coordination complex or polymer thereof. The cobalt coordination complex comprises a cobalt ion chelated by a tetradentate organic chelating ligand.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 15/033,349,filed Apr. 29, 2016, which is the U.S. National Phase of PCT ApplicationNo. PCT/US2014/063724, filed Nov. 3, 2014, which claims the benefit ofProvisional Patent Application No. 61/898,532, filed Nov. 1, 2013, thedisclosures of which are hereby incorporated by reference herein intheir entirety.

GOVERNMENT INTEREST

This invention was made with government support under Grant/ContractNumber EPS1004083 awarded by the National Science Foundation. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

The presently disclosed subject matter relates to catalysts for twoelectron, reversible oxygen reduction. The presently disclosed subjectmatter further relates to air electrodes and electrochemical cellscomprising the catalysts, to methods of making the catalysts, and tomethods of using the catalysts, for example, to produce hydrogenperoxide.

ABBREVIATIONS

-   -   ° C.=degrees Celsius    -   μl=microliters    -   μm=micrometers    -   AEM=anion exchange membrane    -   AgCl=silver chloride    -   Ah=ampere-hour    -   AO=anthraquinone oxidation    -   Au=gold    -   C carbon    -   cm²=square centimeter    -   CV=cyclic voltammetry or cyclic voltammagram    -   DTPA=diethylenetriaminepentaacetic acid    -   EDTA=ethylenediaminetetraacetic acid    -   H₂O₂=hydrogen peroxide    -   IHL=Inner Helmholtz Layer    -   kg=kilogram    -   KOH=potassium hydroxide    -   kWh=kilowatt hour    -   Li₂O=lithium oxide    -   Li₂O₂=lithium peroxide    -   M=molar    -   mA=milliampere    -   mg=milligram    -   ml=microliters    -   mM=millimolar    -   mV/s=milllVolts per second    -   mAh=milliampere-hour    -   O₂=oxygen    -   OER=oxygen evolution reaction    -   OHL=Outer Helmholtz Layer    -   ORR=oxygen reduction reaction    -   Pt=platinum    -   RDE=rotating disk electrode    -   RHE=reversible hydrogen electrode    -   rpm=revolutions per minute    -   RRDE=rotating ring disk electrode    -   salen=N,N′-ethylenebis(salicylimine)    -   SHE=standard hydrogen electrode    -   STEM=scanning transmission electron    -   microscopy    -   V=volts

BACKGROUND

Developing alternative and renewable energy sources is of interest dueto environmental concerns associated with using fossil fuels, as well asthe need for energy security. Several different types of renewableenergy sources have been developed, including wind power, hydropower,solar power, biomass fuels, geothermal energy, and nuclear power.However, some of these energy sources, such as wind and solar power, arenot constant and/or readily transported. Thus, it is often necessary orconvenient to transform the energy source into electrical energy fortransport and then to store it, e.g., as heat with thermal storage or aschemical energy in batteries or capacitors.

In addition, much effort is being made in the automobile industry to tryto replace the combustion engine with clean technology to reducepollution. Possible replacements include two different types ofelectrochemical devices, i.e., fuel cells and batteries. Fuel cells canconvert chemical energy from an environmentally friendly fuel (e.g.,hydrogen) into electricity through electrochemical reaction with oxygen.However, for this approach to be feasible, new methods of fuelproduction, as well as systems for fuel storage and nation-widetransport would need to be developed and/or built.

Batteries and super capacitors transfer and store electrical energy aschemical energy and electrical field energy. In particular, batteriesstore electrical energy as chemical oxidation-reduction energy andconvert this chemical energy back into electrical energy when in use.Chemical energy can be stored in the active materials of a batterynegative electrode. When discharging, electrons travel from the negativeto the positive electrode through an external circuit to power the loadand complete the discharge reaction at the same time as ions flowdirectly from the negative side to the positive side through anintervening electrolyte or electrolytes. In rechargeable batteries (alsoreferred to as secondary batteries), the electrons and ions travel inthe opposite direction during recharge. Given the existence ofestablished electrical grid systems, the use of batteries (e.g.,rechargeable batteries) in the automobile industry could be anattractive option with regard to replacing the combustion engine,assuming that costs, safety, energy density and/or recharging times ofthese devices can be improved.

Lithium-air batteries are currently the subject of much scientificinvestigation due to their high theoretical energy density (i.e., ofabout 12 kWh/kg). When discharging, lithium metal anodes release lithiumions and an electron. The electron goes through an external circuit andlithium ions travel through electrolyte(s) to the cathode. Oxygencombines with the electrons and lithium ions to complete the reactionand produce lithium oxide or lithium peroxide (i.e., Li₂O and Li₂O₂).When recharging, lithium oxide or lithium peroxide decomposes to produceoxygen and lithium ions travel back to the anode and are reduced tolithium metal. The oxygen electrode can be porous, e.g., to store thesolid products generated from the reaction of Li ions with O₂ (i.e.,Li₂O and Li₂O₂) during the discharge cycle of the battery and typicallyinclude a catalyst to promote reactions. Depending upon the type ofelectrolyte(s), four different types of lithium-air battery have beenproposed: aprotic, aqueous, solid state, and mixed aqueous/aprotic. Oneissue, particularly with aprotic lithium-air batteries is that theoverpotential to drive the product back to oxygen and lithium metal islarge, usually over 1.5V.

Accordingly, there is an ongoing need for improved electrochemicalsystems, such as improved metal-air batteries, and for robustbifunctional catalysts (i.e., catalysts that can reduce both the chargeoverpotential and the discharge overpotential) that can be used in suchsystems, e.g., to improve efficiency and recharge rates. For instance,e.g., to help increase system efficiency, there is a need for reversiblecatalysts that can promote both the oxygen reduction reaction (ORR) andthe oxygen evolution reaction (OER).

Hydrogen peroxide is an important commodity chemical. Hydrogen peroxidecan be used as a bleaching agent (e.g., in the paper industry) and as acleaning and disinfecting agent. Hydrogen peroxide has also found use inthe cosmetic industry and as a propellant. Most hydrogen peroxide iscurrently produced via an anthraquinone oxidation process, which resultsin significant chemical waste. Further, since the anthraquinoneoxidation process is difficult to carry out on a small scale, productionof hydrogen peroxide generally requires transport and storage, which canresult in safety concerns. Hydrogen peroxide can also be produced on asmaller scale via direct synthesis from hydrogen and oxygen orelectrochemically. These methods would reduce the need for storage andtransport by allowing hydrogen peroxide to be produced on site, asneeded. The direct synthesis method can involve its own safety concerns(e.g., due to flammability of mixtures of hydrogen and oxygen gases).Recent efforts have been made to improve ORR catalysts for theelectrochemical synthesis of hydrogen peroxide; but there is still aneed for additional, higher performance ORR catalysts for producinghydrogen peroxide.

SUMMARY

In accordance with some embodiments of the presently disclosed subjectmatter provided is a catalyst for two electron, reversible oxygenreduction. In some embodiments, the catalyst comprises: (a) gold; and(b) a cobalt coordination complex or polymer thereof. In someembodiment, the cobalt coordination complex comprises a cobalt ionchelated by a tetradentate organic chelating ligand. In someembodiments, the tetradentate organic chelating ligand isN,N′-bis(salicylidene)ethylenediamine or a derivative thereof.

In some embodiments, the cobalt complex is present in a solution that isin contact with the gold. In some embodiments, the cobalt complex ispresent in a coating covering a surface of a gold structure. In someembodiments, the gold is present in a particle. In some embodiments, theparticle is a nanoparticle, optionally wherein the nanoparticle has adiameter ranging from about 1 nm to about 20 nm.

In some embodiments, the cobalt coordination complex has a structure ofFormula (I):

wherein: each of R₁, R₂, R₃, R₄, R₅, R₇, and R₈ are independentlyselected from the group consisting of H, alkyl, cycloalkyl, aralkyl,aryl, heteroaryl, alkoxy, aryloxy, aralkoxy, thioalkyl, thioaralkyl,thioaryl, aminoalkyl, aminoaralkyl, aminoaryl, and a conducting polymer;and R is alkylene or arylene. In some embodiments, one or more of R₁,R₂, R₃, R₄, R₅, R₆, R₇, and R₈ is a conducting moiety selected from thegroup consisting of thiophenyl, pyrrolyl, —NHC₆H₆, —SC₆H₅; —CH═CH—C₆H₅;—C≡CH, and a conducting polymer. In some embodiments, one or more of R₂,R₃, R₆, and R₇ comprises thiophenyl, optionally wherein both R₃ and R₇are thiophenyl. In some embodiments, R is phenylene or optionallysubstituted ethylene.

In some embodiments, the catalyst comprises a polymer of the cobaltcoordination complex, wherein the polymer is formed by bonds between theorganic chelating ligands of individual monomer units of the cobaltcoordination complex, optionally wherein the polymer is prepared byelectropolymerization. In some embodiments, the polymer is present as afilm coating covering the surface of a gold-containing structure.

In accordance with some embodiments of the presently disclosed subjectmatter provided is an air electrode comprising a catalyst as disclosedherein. In accordance with some embodiments of the presently disclosedsubject matter provided is an electrochemical cell comprising a catalystas disclosed herein, optionally wherein the electrochemical cell is ametal-air battery or a fuel cell.

In accordance with some embodiments of the presently disclosed subjectmatter provided is a method for performing a two electron reduction ofoxygen. In some embodiments, the method comprises contacting oxygen withwater in the presence of a catalyst as disclosed herein, optionallywherein the reduction is reversible or near-reversible.

In accordance with some embodiments of the presently disclosed subjectmatter provided is a method for producing hydrogen peroxide. In someembodiments, the method comprises contacting oxygen with an aqueoussolution in the presence of a catalyst as disclosed herein to providehydrogen peroxide.

In accordance with some embodiments of the presently disclosed subjectmatter provided is a method of preparing a catalyst for two electionoxygen reduction. In some embodiments, the method comprises: (a)providing a solution comprising a cobalt coordination complex,optionally wherein the cobalt coordination complex comprises a cobaltion chelated by a tetradentate organic chelating ligand, optionallywherein the tetradentate organic chelating ligand isN,N′-bis(salicylidene)ethylenediamine or a derivative thereof; (b)providing a structure comprising a gold surface; (c) contacting thestructure with the solution from step (a); (d) electropolymerizing thecobalt coordination complex to form an electropolymerized film coveringat least a portion of the gold surface.

It is an object of the presently disclosed subject matter to providecatalysts for two electron oxygen reduction (e.g., reversible oxygenreduction), as well as to provide electrodes and electrochemical cellscomprising the catalysts, methods of producing the catalysts, andmethods of using the catalysts to reduce oxygen and/or provide hydrogenperoxide.

An object of the presently disclosed subject matter having been statedhereinabove, and which is achieved in whole or in part by the presentlydisclosed subject matter, other objects will become evident as thedescription proceeds when taken in connection with the accompanyingdrawings as best described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the cyclic voltammetry (CV) comparison of anormal (unmodified) gold (AU) electrode (dotted line) and a cobalt salenmodified gold electrode (solid line) in 1 Molar (1 M) potassiumhydroxide (KOH) electrolyte saturated with oxygen gas. The scan rate was50 milliVolts per second (mV/s). The electrode surface area was 0.19625square centimeters (cm²). The reference electrode was sliver chloride(AgCl) and the counter electrode was gold wire.

FIG. 2 is a graph showing the tafel plots for oxygen reduction on arotating disk polycrystalline gold electrode in 1 Molar (1 M) potassiumhydroxide (KOH) electrolyte and in an electrolyte comprising 1 Molar KOHand 2 milliMolar (mM) cobalt salen. The scan rate was 10 milliVolts persecond (mV/s); the revolutions per minute (rpm) of the electrode was1000; the electrode surface area was 0.19625 square centimeters (cm²);the reference electrode was silver chloride (AgCl); and the counterelectrode was gold wire.

FIG. 3A is a schematic drawing showing a stretched oxygen molecule (O₂)bonded to cobalt salen In the Outer Helmholtz Layer (OHL) of a gold (Au)surface can be transferred to the Inner Helmholtz Layer (IHL) on thesurface. The drawing also shows water molecules (H₂O) and hydroxide ions(OH) associated with the OHL and IHL.

FIG. 3B is a schematic drawing showing how electron transfer via atunneling phenomenon from a gold (Au) surface to an oxygen molecule (O₂)bonded to cobalt salen on the surface of the gold. The drawing alsoshows water molecules (H₂O) and hydroxide ions (OH) associated with theOuter Helmholtz Layer (OHL) and the Inner Helmholtz Layer (IHL) of theAu surface.

FIG. 4A is a graph showing the cyclic voltammetry (CV) comparison of thecatalytic activity of a gold electrode with free cobalt salen in anelectrolyte comprising 1 Molar (1M) potassium hydroxide (KOH) saturatedwith oxygen after 3, 10, 20, and 30 cycles. The scan rate was 50milliVolts per second (mV/s). The reference electrode was silverchloride (AgCl) and the counter electrode was gold wire.

FIG. 4B is a graph showing the cyclic voltammetry (CV) comparison of thecatalytic activity of a gold electrode having electropolymerized cobaltsalen at the surface in an electrolyte comprising 1 Molar (1 M)potassium hydroxide (KOH) saturated with oxygen after 1, 30, 50, 100,150, 200, and 250 cycles. The scan rate was 50 milliVolts per second(mV/s). The reference electrode was silver chloride (AgCl) and thecounter electrode was gold wire.

FIG. 5 is a cyclic voltammagram (CV) of electropolymerized thiophenemodified cobalt salen on a gold electrode in a 1 Molar (1 M) potassiumhydroxide (KOH) electrolyte saturated with oxygen. The scan rate was 50millVolts per second (mV/s). The reference electrode was silver chloride(AgCl) and the counter electrode was gold wire.

FIG. 6 is a graph showing the relationship between oxygen reductionreaction (ORR) current density and two electron ORR selectivity using agold/cobalt salen catalyst (cobalt salon film on a gold rotating ringdisk electrode (RRDE)) in a 1 Molar (1 M) potassium hydroxide (KOH)electrolyte. The reference electrode was a reversible hydrogen electrode(RHE).

FIG. 7A Is a graph comparing the oxygen reduction reaction (ORR) currentdensity (milliampere/square centimeters (mA/cm²)) of a conventionalperoxide producing catalyst (i.e., carbon black), a gold/cobalt salencatalyst, and a typical proton exchange membrane fuel cell catalyst(i.e., platinum (Pt) on carbon). The ORR was performed using a rotatingdisk electrode: 10 milliVolts per second (mV/s); 1000 revolutions perminute (rpm); and using a reversible hydrogen electrode (RHE) in 1 Molar(1 M) potassium hydroxide (KOH) as the electrolyte.

FIG. 7B is a graph of the tafel plots from the catalysts described forFIG. 7A.

FIG. 8A is a schematic drawing showing electrolytic cells for hydrogenperoxide (H₂O₂) production according to an embodiment of the presentlydisclosed subject matter. The cell on the left-hand side is anelectrolyser that includes an anode (comprising a nickel mesh oxygenevolution catalyst), an anion exchange membrane, and an air electrode(cathode) that comprises a gas diffusion electrode that comprises acatalyst of the presently disclosed subject matter. Hydrogen peroxidecan be removed via the cathode. The cell on the right-hand side is afuel cell that comprises a hydrogen source associated with an anode anda cathode comprises a gas diffusion electrode that comprises a catalystof the presently disclosed subject matter. An aqueous alkalineelectrolyte flows through the cell removing hydrogen peroxide, which canbe dissolved in the electrolyte.

FIG. 8B is a graph of the current efficiency versus the electrolyticcell operation voltage for the electrolyser of FIG. 8A.

FIG. 9A is a schematic drawing showing a reversible zinc air batteryaccording to an embodiment of the presently disclosed subject matter.The battery includes an anion exchange membrane disposed between aporous zinc electrode and an air electrode. The air electrode isassociated with a layer of gold/cobalt salen catalyst. An aqueouselectrolyte (not shown) can be in contact with the catalyst layer andthe anion exchange membrane. The battery is configured to transportaqueous hydrogen peroxide (H₂O₂ in H₂O) solution out of the battery forstorage and back into the battery during recharging.

FIG. 9B is a graph of the charge (circles) and discharge (squares)polarization curves of the zinc air battery described for FIG. 9A FIG.10A is a schematic drawing showing a discharge diagram of a lithium airbattery according to an embodiment of the presently disclosed subjectmatter. The battery comprises a flowing electrolyte structure and ispart of a system that further comprises a unit for the separation andstorage of lithium peroxide (Li₂O₂) outside of the battery cell. Thecell also includes a catalyst layer in contact with the flowing aqueouselectrolyte and the air electrode, as well as a lithium ion (Li⁺)conducting solid membrane disposed between the aqueous electrolyte andthe lithium anode.

FIG. 10B is a schematic drawing showing a charge diagram of the lithiumair battery of FIG. 10A.

FIG. 11 is a schematic drawing showing a lithium air battery structureaccording to another embodiment of the presently disclosed subjectmatter. The flow of anions is shown for the battery discharge. Lithiumanions (Li⁺) are conducted from a protected lithium anode though aporous support containing an organic electrolyte and also through asolid state electrolyte to an aqueous alkaline electrolyte. The aqueousalkaline electrolyte is in contact with a catalyst layer that isassociated with the gas diffusion layer (GDL) of an air electrode.Oxygen from the air electrode is reduced to hydroperoxide (HO₂) by thecatalyst. The alkaline electrolyte can flow through the cell (see thickgrey arrows) via an Inlet and outlet In the cell. Current collectors anda circuit for current flow are also shown.

FIG. 12 is a schematic drawing showing a lithium air battery structureaccording to another embodiment of the presently disclosed subjectmatter, wherein a safety procedure is provided to control the batteryexplosion hazard to minimum when receive outside collision accident

FIG. 13 is a graph showing a comparison of ORR CV on (Au/C-a) catalyst(a. without cobalt salen b. with cobalt salen in the preparation ink) inO2 saturated 1M KOH solution.

FIG. 14 is a graph showing CVs of various scan rates on Au (poly)electrode in a reduced scan range particular for oxygen reduction redoxpeaks. Electrolyte: 1 M KOH+1 mM cobalt salen, IR corrected.

DETAILED DESCRIPTION

The presently disclosed subject matter will now be described more fully.The presently disclosed subject matter can, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein below and in the accompanying Examples.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of theembodiments to those skilled in the art.

All references listed herein, including but not limited to all patents,patent applications and publications thereof, and scientific Journalarticles, are incorporated herein by reference in their entireties tothe extent that they supplement, explain, provide a background for, orteach methodology, techniques, and/or compositions employed herein.

I. Definitions

While the following terms are believed to be well understood by one ofordinary skill in the art, the following definitions are set forth tofacilitate explanation of the presently disclosed subject matter.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which the presently disclosed subject matter belongs.

Following long-standing patent law convention, the terms “a”, “an”, and“the” refer to “one or more” when used in this application, includingthe claims.

The term “and/or” when used in describing two or more items orconditions, refers to situations where all named Items or conditions arepresent or applicable, or to situations wherein only one (or less thanall) of the items or conditions is present or applicable.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive, although the disclosure supports a definitionthat refers to only alternatives and “and/or.”

As used herein “another” can mean at least a second or more.

The term “comprising”, which is synonymous with “including,”“containing,” or “characterized by” is Inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the namedelements are essential, but other elements can be added and still form aconstruct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”appears in a clause of the body of a claim, rather than immediatelyfollowing the preamble, it limits only the element set forth in thatclause; other elements are not excluded from the claim as a whole.

As used herein, the phrase “consisting essentially of” limits the scopeof a claim to the specified materials or steps, plus those that do notmaterially affect the basic and novel characteristic(s) of the claimedsubject matter.

With respect to the terms “comprising”, “consisting of”, and “consistingessentially of”, where one of these three terms is used herein, thepresently disclosed and claimed subject matter can include the use ofeither of the other two terms.

Unless otherwise indicated, all numbers expressing quantities oftemperature, pH, weight percentage (%), and so forth used in thespecification and claims are to be understood as being modified in allInstances by the term “about”. Accordingly, unless indicated to thecontrary, the numerical parameters set forth in this specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the presently disclosedsubject matter.

As used herein, the term “about”, when referring to a value is meant toencompass variations of in one example ±20% or ±10%, In another example±5%, in another example ±1%, and in still another example ±0.1% from thespecified amount, as such variations are appropriate to perform thedisclosed methods.

As used herein the term “alkyl” refers to C₁₋₂₀ Inclusive, linear (i.e.,“straight-chain”), branched, saturated or at least partially and in somecases fully unsaturated (i.e., alkenyl and alkynyl) hydrocarbon chains,including for example, methyl, ethyl, propyl, isopropyl, butyl,isobutyl, tert-butyl, pentyl, hexyl, octyl, ethenyl, propenyl, butenyl,pentenyl, hexenyl, octaenyl, butadienyl, propynyl, butynyl, pentynyl,hexynyl, heptynyl, and allenyl groups. “Branched” refers to an alkylgroup in which a lower alkyl group, such as methyl, ethyl or propyl, isattached to a linear alkyl chain. “Lower alkyl” refers to an alkyl grouphaving 1 to about 8 carbon atoms (i.e., a C₁₋₈ alkyl), e.g., 1, 2, 3, 4,5, 6, 7, or 8 carbon atoms. “Higher alkyl” refers to an alkyl grouphaving about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15,16, 17, 18, 19, or 20 carbon atoms.

Alkyl groups can optionally be substituted (a “substituted alkyl”) withone or more alkyl group substituents, which can be the same ordifferent. The term “alkyl group substituent” includes but is notlimited to alkyl (saturated or unsaturated), substituted alkyl (e.g.,halo-substituted and perhalo-substituted alkyl, such as but not limitedto, —CF₃), cycloalkyl, halo, nitro, hydroxyl, carbonyl, carboxyl, acyl,alkoxyl, aryloxyl, aralkoxyl, thioalkyl, thioaryl, thioaralkyl, amino(e.g., aminoalkyl, aminodialkyl, aminoaryl, etc.), sulfonyl, andsulfinyl.

The term “aryl” is used herein to refer to an aromatic substituent thatcan be a single aromatic ring, or multiple aromatic rings that are fusedtogether, linked covalently, or linked to a common group, such as, butnot limited to, a methylene or ethylene moiety. The common linking groupalso can be a carbonyl, as in benzophenone, or oxygen, as indiphenylether. Thus, examples of aryl include, but are not limited to,phenyl, naphthyl, biphenyl, and diphenylether, among others. Aryl groupsinclude heteroaryl groups, wherein the aromatic ring or rings Include aheteroatom (e.g., N, O, S, or Se). Exemplary heteroaryl groups Include,but are not limited to, furanyl, pyridyl, pyrimidinyl, imidazoyl,benzimidazolyl, benzofuranyl, benzothiophenyl, quinolinyl,isoquinolinyl, and thiophenyl.

The aryl group can be optionally substituted (a “substituted aryl”) withone or more aryl group substituents, which can be the same or different,wherein “aryl group substituent” includes alkyl (saturated orunsaturated), substituted alkyl (e.g., haloalkyl and perhaloalkyl, suchas but not limited to —CF₃), cycloalkyl, aryl, substituted aryl,aralkyl, halo, nitro, hydroxyl, acyl, carboxyl, alkoxyl (e.g., methoxy),aryloxyl, aralkyloxyl, thioalkyl, thioaryl, thioaralkyl, amino (e.g.,aminoalkyl, aminodialkyl, aminoaryl, etc.), sulfonyl, and sulfinyl.

“Alkylene” refers to a straight or branched bivalent aliphatichydrocarbon group having from 1 to about 20 carbon atoms, e.g., 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbonatoms. The alkylene group can be straight, branched or cyclic. Thealkylene group also can be optionally unsaturated and/or substitutedwith one or more “alkyl group substituents.” There can be optionallyInserted along the alkylene group one or more oxygen, sulfur orsubstituted or unsubstituted nitrogen atoms (also referred to herein as“alkylaminoalkyl”), wherein the nitrogen substituent is alkyl aspreviously described. Exemplary alkylene groups include methylene(—CH₂—); ethylene (—CH₂—CH₂—); propylene (—(CH₂)₃—); cyclohexylene(—C₆H₁₀—); —CH═CH—CH═CH—; —CH═CH—CH₂—; —(CH₂)_(q)—N(R)—(CH₂)_(r)—,wherein each of q and r is Independently an integer from 0 to about 20,e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, or 20, and R is hydrogen or lower alkyl; methylenedioxy (—O—CH₂—O—);and ethylenedioxy (—O—(CH₂)₂—O—). An alkylene group can have about 2 toabout 3 carbon atoms and can further have 6-20 carbons.

The term “arylene” refers to a bivalent aromatic group, e.g., a bivalentphenyl or napthyl group. The arylene group can optionally be substitutedwith one or more aryl group substituents and/or include one or moreheteroatoms.

“Aralkyl” refers to an aryl-alkyl- or an -alkyl-aryl group wherein aryland alkyl are as previously described, and can include substituted aryl,heteroaryl, and substituted alkyl. Thus, “substituted aralkyl” can referto an aralkyl group comprising one or more alkyl or aryl groupsubstituents. Exemplary aralkyl groups include benzyl, phenylethyl, andnaphthylmethyl.

“Cyclic” and “cycloalkyl” refer to a non-aromatic mono- or multi-cyclicring system of about 3 to about 10 carbon atoms, e.g., 3, 4, 5, 6, 7, 8,9, or 10 carbon atoms. The cycloalkyl group can be saturated orpartially unsaturated. The cycloalkyl group also can be optionallysubstituted with an alkyl group substituent as defined herein. There canbe optionally inserted along the cyclic alkyl chain one or more oxygen.Representative monocyclic cycloalkyl rings Include cyclopentyl,cyclohexyl, and cycloheptyl. Multicyclic cycloalkyl rings includeadamantyl, octahydronaphthyl, decalin, camphane, and noradamantyl.

“Alkoxyl” and “alkoxy” refer to an alkyl-O— group wherein alkyl is aspreviously described, including substituted alkyl. The term “alkoxyl” asused herein can refer to, for example, methoxyl, ethoxyl, propoxyl,isopropoxyl, butoxyl, t-butoxyl, and pentoxyl. The term “oxyalky” can beused interchangably with “alkoxyl”.

“Aryloxyl” and “aryloxy” refer to an aryl-O— group wherein the arylgroup is as previously described, including a substituted aryl. The term“aryloxyl” as used herein can refer, for example, to phenyloxyl and toalkyl, substituted alkyl, or alkoxyl substituted phenyloxyl.

“Aralkyloxyl”, “aralkoxyl”, and “aralkoxy” refer to an aralkyl-O— groupwherein the aralkyl group is as previously described. An exemplaryaralkyloxyl group is benzyloxyl. “Substituted aralkyoxyl” can refer toan aralkoxyl group wherein the alkyl and/or aryl portion of the aralkylare substituted by one or more alkyl or aryl group substituents.

The term “vinyl” can refer to the group —CH═CH₂, optionally wherein oneor more of the hydrogen atoms is replaced by an alkyl groupsubstitutent. Thus, vinyl can refer to substituted or unsubstitutedvinyl. “Vinylphenyl” refers to the group —CH═CH-phenyl or —CH═CH—CH₅.

The terms “halo”, “halide”, or “halogen” as used herein refer to fluoro,chloro, bromo, and iodo groups.

The term “amino” refers to the group —N(R)₂ wherein each R isindependently H, alkyl, substituted alkyl, aryl, substituted aryl,aralkyl, or substituted aralkyl. The terms “aminoalkyl” and “alkylamino”can refer to the group —N(R)₂ wherein each R is H, alkyl or substitutedalkyl, and wherein at least one R is alkyl or substituted alkyl.“Arylamine” and “aminoaryl” refer to the group —N(R)₂ wherein each R isH, aryl, or substituted aryl, and wherein at least one R is aryl orsubstituted aryl, e.g., aniline (i.e., —NHC₆H₅).

The term “thioalkyl” can refer to the group —SR, wherein R is selectedfrom H, alkyl, substituted alkyl, aralkyl, substituted aralkyl, aryl,and substituted aryl. Similarly, the terms “thioaralkyl” and “thioaryl”refer to —SR groups wherein R is aralkyl and aryl, respectively.

A “coordination complex” is a compound in which there is a coordinatebond between a metal ion and an electron pair donor, ligand or chelatinggroup. Thus, ligands or chelating groups are generally electron pairdonors, molecules or molecular ions having unshared electron pairsavailable for donation to a metal ion.

The term “coordinate bond” refers to an interaction between an electronpair donor and a coordination site on a metal ion resulting in anattractive force between the electron pair donor and the metal ion. Theuse of this term is not intended to be limiting, in so much as certaincoordinate bonds also can be classified as have more or less covalentcharacter (if not entirely covalent character) depending on thecharacteristics of the metal ion and the electron pair donor.

As used herein, the term “ligand” refers generally to a species, such asa molecule or ion, which interacts, e.g., binds, in some way withanother species: More particularly, as used herein, “ligand” can referto a molecule or ion that binds a metal ion in solution to form a“coordination complex.” See Martell, A. E., and Hancock, R. D., MetalComplexes in Aqueous Solutions, Plenum: New York (1996), which isincorporated herein by reference in its entirety. The terms “ligand” and“chelating group” can be used interchangeably. Organic ligands can havetwo or more groups with unshared electron pairs separated by, forexample, an alkylene or arylene group. Groups with unshared electronpairs, include, but are not limited to, —CO₂H, —NO₂, —B(OH)₂, —SO₃H,PO₃H, phosphonate, and heteroatoms (e.g., nitrogen) in heterocycles.

“Tetradentate” as used herein refers to a chelating ligand that formsfour coordinate bonds with a metal ion.

“Bifunctional” as used herein refers to a catalyst that can catalyzeboth oxygen reduction reactions and oxygen evolution reactions and/orthat can reduce both the charge overpotential and the dischargeoverpotential.

As used herein, the term “reversible” can refer to both chemical andelectrochemical reversibility. For a chemically reversible catalyst ofthe presently disclosed subject matter, the oxidation peak currentdensity is equal to the reduction peak current density (i.e., the ratioof peak reduction current density/peak oxidation current density is 1)and all of the oxygen that is reduced can be oxidized back.Electrochemical reversibility can refer to the rate at which electrontransfer occurs between a working electrode and redox species insolution, particularly that the electron transfer should occur quickly.A large rate constant for electron transfer (e.g., a rate constant, k₅,of greater than about 0.020 cm/s) can be indicative of electrochemicalreversibility.

The term “near-reversible” refers to a catalyst of the presentlydisclosed subject matter that is close to being reversible, eitherchemically, electrochemically or both. With regard to chemicalreversibility, the ratio of peak reduction current density/peakoxidation current density is between about 1.001 and about 1.11 and/orabout 90% or more (e.g., 90, 95, 96, 97, 98, or 99% or more of theoxygen that is reduced can be oxidized back. With regard toelectrochemical reversibility, the rate constant k_(s) can be betweenabout 0.20 cm/s and about 0.00005 cm/s. Also, the shape of the CV canindicate a highly reversible case. Referring to FIG. 14, the peakseparation is about 36 mV, this should be most probably a two electronredox reaction. Secondly, in the example of a two electron redoxprocess, the separation peak potential difference, 36 mV≈58 mV/2 showsthis is a nearly reversible case. Continuing with FIG. 14, the oxidationpeak current is slightly smaller than the reduction peak current, theratio of peak reduction current density over peak oxidation currentdensity is 1:0.81. This also proves that this is a quasi-reversible caseand the cathodic reduction product has a EC mechanism.

Continuing with FIG. 14, variation of scan rate is a basic but veryuseful method in CV. By changing the scan rate, the experiment isaltering the balance between the speed of electron transfer and reactanttransport. The difference between a reversible redox process and aquasi-reversible redox process can be shown in the speed of hetergeneouselectron transfer. For the reversible case, no matter what the scan rateis, the hetergeneous electron transfer rate is fast enough to reachequilibrium. Thus, the CV's peak potential will not change along withthe scan rate change, which is one standard to verify whether the redoxcouple is electrochemical reversible or not. From FIG. 14, we can seethat the peak potential separation almost does not shift with one orderof magnitude increase on scan rate. This is reversible case according tothis standard.

The term “electrochemical cell” refers to system that can generateelectricity from chemical reactions or that can facilitate a chemicalreaction using electrical energy. In some embodiments of the presentlydisclosed subject matter, electrochemical cells can include at least twoelectrodes and an electrolyte. Exemplary electrochemical cells include,but are not limited to, metal air cells (e.g., metal air batteries),such as zinc air cells, aluminum air cells, magnesium air cells, sugarair cells, and the like, that use air electrodes (i.e., oxygen-reducingelectrodes); fuel cells (e.g., oxygen hydrogen fuel cells, directmethanol fuel cells and the like), electrolysers (e.g., peroxideproducing electrolysers), and electrochemical sensors (e.g., enzymesensors, oxygen sensors, protein sensors, etc.).

The term “electrolyte” refers to a material that can conduct ions.Typically, electrolytes are liquids or solids (e.g., gels, polymers, orceramics). Liquid electrolytes can be aqueous or based on an organicsolvent and can further include one or more ion conducting salts.

The term “battery” refers to device that comprises one or moreelectrochemical cell or cells that can convert stored chemical energyinto electrical energy. The term “primary battery” refers to adisposable, single use battery wherein the electrode materials areirreversible changed during discharge; while the term “secondarybattery” refers to a battery that can discharge and be rechargedmultiple times and wherein the original electrode materials can berestored by during recharge.

II. General Considerations

Metal air batteries have been known for over a century, but haverecently garnered attention in view of demands for new high energydensity batteries. Metal air batteries, which utilize oxygen as apositive electrode active material, can combine high energy density withreduced size and weight. Examples of metal air batteries include, forexample, lithium air batteries, magnesium air batteries, and zinc airbatteries.

Metal air batteries can charge/discharge by performing a redox reactionof oxygen in an air electrode (which can be, for example, a gasdiffusion electrode (GDE)) and a redox reaction of a metal contained ina metal electrode. Thus, metal air batteries can include an airelectrode, which can include an air electrode current collector thatcollects a current of the air electrode; a negative electrode thatcontains a negative electrode active material (i.e., a metal or metalalloy) and which can contain a negative electrode current collector thatcollects a current of the negative electrode; and one or moreelectrolytes provided between the air electrode and the negativeelectrode. Unlike other batteries, the positive electrode activematerial is not stored in the battery. Rather, when the battery isexposed to the environment, oxygen can enter the battery, e.g., throughan oxygen diffusion membrane and a porous air electrode. Typically, acatalyst is associated with the air electrode to catalyze reduction ofthe oxygen, e.g., at a surface of the air electrode.

The most extensively developed kinds of air electrode for rechargeablebatteries to date include conventional aqueous air electrodes andaprotic air electrodes. The conventional aqueous air electrode is basedon four electron irreversible oxygen reduction reaction (ORR) chemistry:O₂(g)+4e ⁻⁺²H₂⇄4OH⁻A regenerative fuel cell using this air electrode chemistry can have acell discharge voltage and charge voltage as below:

Cell discharge voltage: 0.9V Cell recharge voltage: 1.43V

Conventional aqueous air electrodes are mainly studied and used in zincair batteries and in regenerative fuel cell systems. A challenge forthis type of rechargeable air electrode is the large voltage gap betweencharging and discharging because of the irreversible nature of thechemistry, making it difficult to find a bifunctional catalyst that canreduce the voltage gap while still remaining stable and inexpensive.While some improvements have been achieved, the system efficiency isstill around 50% to 65% at reasonable charge and discharge rates.

Aprotic air electrodes can provide a wide electrochemical window andkeep battery structure simple. However, the lithium peroxide or lithiumsuperoxide (LiO₂) generated during oxygen reduction is very reactivetoward electrolyte and binder materials, and many side reactions canhappen along with the intended air electrode chemistry. Furthermore,oxygen containing discharge products are electron and lithium ioninsulating. Thus, high current density discharge can cause cloggingproblems and damage porous support structures. Accordingly, the furtheruse of this kind of air battery will likely focus on low power densityand long term energy storage applications. In addition, provedreversible air electrode chemistry in non-aqueous electrolyte haspreviously been exclusively one electron reaction with superoxide as adischarge product, which does not release the full potential of the airelectrode.

In accordance with some embodiments, the presently disclosed catalystscan catalyze reversible two electron ORR:O₂+H₂O+2e ⁻⇄HO₂ ⁻+OH⁻A regenerative fuel cell based on this air electrode chemistry can havecharge and discharge cell voltage as below:

Cell discharge voltage: 0.83V Cell charge voltage: 0.83V

By “reversible” is meant both chemically and electrochemicallyreversible. Thus, the system voltage efficiency can reach a high ratio.Further, the catalyst has a high kinetic performance separately on twoelectron ORR and two electron OER. Moreover, producing peroxide inaqueous solution can provide various air battery features not previouslypossible. Since the peroxide is produced into aqueous solution, it canbe possible to separate the energy density and power density.

Hydrogen peroxide (H₂O₂) is an important commodity chemical. Demand forhydrogen peroxide has grown significantly due to its “green” character.However, at present, hydrogen peroxide is mainly produced on anindustrial scale by the anthraquinone oxidation (AO) process, which isnot considered a green method. The AO process involves the sequentialhydrogenation and oxidation of an alkylanthraquinone precursor dissolvedin a mixture of organic solvents. The process generates significantchemical waste and is difficult to carry out in small scale. Inaddition, the transport, storage, and handling of bulk hydrogen peroxideinvolve hazards and further expense. Therefore, a direct onsitesynthesis method for hydrogen peroxide is desirable.

Direct synthesis of H₂O₂ from H₂ and 0₂ has been studied for a centurysince the first patent application filed by Henkel and Weber in 1914.However, little progress has been made because of safety issues (H₂-0₂mixture are flammable over the range of 4-94% H₂ by volume). Theexplosion hazards can be avoided by diluting H₂/0₂ feed streams awayfrom the explosion limits or by using membrane-based reactors toseparate the two gases; but both of those methods have a low reactionrate and are not practical for commercial purposes. On the other hand,having 0₂ and H₂ react efficiently with a separator in the middle hasbeen further developed by recent research in fuel cells. Theelectrochemical direct synthesis method has many advantages over anormal chemical direct synthesis method. A comparison of the AO process,direct synthesis and electrochemical direct synthesis is shown in Table1.

TABLE 1 Comparison between AO process, chemical direct synthesis andelectrochemical direct synthesis. Indirect synthesis ElectrochemicalPROCESS AO process Direct synthesis Direct synthesis PRINCIPLESequential H₂ + O₂ = H₂O₂ + H₂ +O₂ = H₂O₂ + Hydrogenation and heatheat + electricity oxidation of organic or mole electricity + H₂O + O₂ =H₂O₂ GENERAL Mature, well known, Simple, small scale, Simple, smallscale FEATURE large scale low CATALYST Pd in hydrogenation Pd- and Au-based Mostly carbon step catalyst REACTION RATE Slow, several steps SlowFast (With presently Disclosed catalyst) REACTOR A complex system Singlereactor Single reactor SYSTEM with numerous reactors ABILITY TO USECapable Not capable Capable AIR High Is an issue High (With presentlySELECTIVITY disclosed catalyst) SAFETY Safe Is an issue Safe ON SITEImpossible Possible Possible PRODUCTION

Depending upon which chemistry is operated in the other electrode (i.e.,the non-air electrode), electrochemical direct synthesis can have twomodes on producing hydrogen peroxide: fuel cell mode and electrolyticmode. Fuel cell direct synthesis can be seen as a premium version of amembrane based chemical reactor. Not only are the two reactantsseparated by the electrolyte, but the mass transfer within theelectrolyte can also be faster than a simple membrane reactor. This isbecause the reactant transport is not limited by gas diffusion throughthe membrane to finish the reaction. For a fuel cell system, part of thechemical reaction also generates electricity by a direct conversionprocess, which is more efficient than through a thermal process. The GDEdeveloped along with fuel cell research also provides for the use oxygenin air from the atmosphere, which is hard to achieve in normal chemicaldirect synthesis. Therefore, fuel cell direct synthesis can be a goodmethod for onsite hydrogen peroxide production.

Another electrochemical hydrogen peroxide direct synthesis methodinvolves the use of an oxygen evolution reaction electrode to replacethe hydrogen electrode of a fuel cell and to operate the cell in anelectrolytic mode. This can be beneficial in locations where usingelectricity is more cost-effective than using hydrogen. Both fuel celland electrolytic direct synthesis benefit from using advancedcatalyst-based air electrodes to generate hydrogen peroxide.

There has already been some effort from both industry and academicresearch on electrochemical hydrogen peroxide onsite production. Forinstance, processes have been described which employ cathodes fabricatedfrom a bed of reticulated vitreous carbon covered by carbon powder/PTFEcomposite or GDEs. These efforts have made strides in solving theproblem of low solubility of oxygen in aqueous solutions fed into thecell. However, commercialization attempts have failed because thehydrogen peroxide produced in these methods is relatively higher inprice than hydrogen peroxide produced by other methods due to the use oflow performance catalysts. Electrochemical systems based on lowperformance catalysts will have low system efficiency. For a peroxideproducing fuel cell system, this means much less electrical energy canbe produced with same amount of reactant since more energy is lost inoverpotential to drive the kinetics, which will increase the cost ofhydrogen peroxide. For an electrolytic cell, this means more electricityis needed to produce hydrogen peroxide. Also, for both productionmethods, a much larger cell is needed for the same hydrogen peroxideproduction rate. This is because the less efficient operating point alsoleads to a relatively low operating current density, which in turnresults in a higher hardware cost per unit hydrogen peroxide produced.

A catalyst with better kinetic performance and high selectivity for twoelectron oxygen reduction would be advantageous in further developingboth metal-air batteries and fuel cell direct synthesis processes forproducing hydrogen peroxide. However, in general, known electrocatalysisof ORR so far either involves fairly high kinetic performance withfour-electron oxygen reduction or relatively poor kinetic performancewith a two-electron oxygen reduction. Although some effort has beenexpended to explore advanced catalysts, carbon still remains as the mostused catalyst. Carbon, well known as a catalyst support in manydifferent kinds of electrochemical systems, has very low electrochemicalkinetic performance for two electron ORR to produce hydrogen peroxide.Other catalysts with slightly improved performance have not been able toovercome the advantage of carbon on cost.

The presently disclosed subject matter is based at least in part on thefinding of a new catalyst for two electron ORR that comprises gold (Au)and cobalt salen (or a derivative and/or polymer thereof). As describedin the examples, catalysts made of Au and cobalt salen can promoteoxygen reduction with higher kinetics and onset potential than carbon,while still retaining over 90% selectivity for two electron oxygenreduction over a wide potential range. See, for example, FIGS. 6 and 7A.Preliminary peroxide production experiments with a simpleelectrochemical cell structure show that the presently disclosedcatalyst has two times higher current under the same experimentalcondition than a normal carbon/PTFE composite catalyst without losingany current efficiency. This means that nearly two times as muchhydrogen peroxide is generated with the presently disclosed catalysts atthe same voltage and experimental conditions. Furthermore, not only is alarge improvement over carbon on two electron ORR observed, but, asshown in FIG. 7B, the kinetic performance of the presently disclosedcatalyst is also comparable with Pt, which is the most commonly used ORRcatalyst in high performance fuel cell systems.

The importance of a high performance catalyst is not only about thecatalyst itself, but also provides for the use of high performance fuelcell flow field design. From recent fuel cell research, flow fielddesign for high current density fuel cells has been significantlyadvanced. However, this knowledge is not as useful if the peroxideproducing fuel cell/electrolyser is built based on a low performancecarbon ORR catalyst. Based on the presently disclosed high performancetwo electron ORR catalysts, existing knowledge on flow field design canbe adapted to build a better high performance and highly efficient fuelcell/electrolyser to produce hydrogen peroxide. As described furtherhereinbelow, using an electrolytic cell as shown in FIG. 8A, theperoxide producing current efficiency of the presently disclosedcatalysts can be over 90% in a wide potential range.

III. Two Electron, Reversible ORR Catalysts

Accordingly, in some embodiments, the presently disclosed subject matterprovides a catalyst for two electron oxygen reduction. The catalyst canbe reversible or near-reversible. The catalyst comprises a cobaltcoordination complex and gold. The cobalt coordination complex comprisesa cobalt ion chelated by an organic chelating ligand, e.g., an organictetradentate chelating ligand, or a polymer thereof.

In some embodiments, the organic chelating ligand isN,N′-bis(salicylidene)ethylenediamine or a derivative or polymerthereof. Thus, in some embodiments, the cobalt coordination complex isN,N′-Bis(salicylidene)ethylenediaminocobalt (II) (i.e., cobalt salen,which is also sometimes referred to as ethylenebis(salicylimine)cobalt(II))

In some embodiments, cobalt coordination complex can comprisederivatives of N,N′-bis(salicylidene)ethylenediamine. In someembodiments, the cobalt coordination complex has a structure of Formula(I):

wherein:

each of R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈ are independently selectedfrom the group comprising H, alkyl, cycloalkyl, aralkyl, aryl,heteroaryl, alkoxy, aryloxy, aralkoxy, thioalkyl, thioaralkyl, thioaryl,aminoalkyl, aminoaralkyl, aminoaryl, and a conducting polymer; and

R is an alkylene or arylene group.

In some embodiments, one of R₁-R₈ is a conducting moiety, such as aconducting polymer (i.e., a monovalent group formed from a conductingpolymer or oligomer). A variety of conducting polymers are known in theart, including but not limited to, polyacetylene, polyphenylenevinylene, polypyrrole, polythiophene (e.g.,poly(3,4-ethylenedioxythiophene) (PEDOT)), polyaniline, andpolyphenylene sulfide. Suitable conducting moieties can also be selectedfrom groups based on the monomers of these polymers, such as thiophenyl(i.e., a monovalent substituent derived from thiophene), pyrrolyl,—NHC₆H₅, —SC₆H₅; —CH═CH—C₆H₅; and —C≡CH. In some embodiments, theconducting moiety or conducting polymer can also be substituted on thealkylene or arylene R group of the coordination complex. In someembodiments, one or more of R₂, R₃, R₆, and R₇ or one or both of R₃ andR₇ is/are a conducting moiety or polymer.

In some embodiments, one or more of R₁-R₈ is alkoxy, alkoxy-substitutedphenyl, thiophenyl, or benzothiophenyl. For example, one or more ofR₁-R₈ can be: methoxy,

In some embodiments, the coordination complex is a thiophene modifiedcobalt salen wherein one or more of R₁₋₈ is thiophenyl. In someembodiments, one or more of R₂, R₃, R₆, and R₇ is thiophenyl. In someembodiments, R₃ and R₇ are thiophenyl. Thus, in some embodiments, thecobalt coordination complex is a thiophene modified cobalt salen havingthe structure:

The R group alkylene or arylene group can be optionally substitutedand/or include one or more heteroatoms (e.g., be heteroarylene). In someembodiments, R is alkylene optionally having one or more oxygen atomsinserted in the carbon chain (e.g., R can include one or more—O—CH₂CH₂—O— units), heteroarylene (e.g., a divalent pyridyl ortriazolyl group), phenylene, or cyclic alkylene (e.g., divalentcyclohexyl). In some embodiments, R is ethylene or optionallysubstituted ethylene, e.g., the group-CH(R₉)CH(R₁₀)—, wherein R₉ and R₁₀are independently selected from H, alkyl cycloalkyl, aralkyl,heteroaryl, aryl, alkoxy, aralkoxy, aryloxy, thioalkyl, thioaryl,thioaralkyl, aminoalkyl, aminoaralkyl, and aminoaryl, or wherein R₉ andR₁₀ together form an alkylene or arylene group. Thus, in someembodiments, the cobalt coordination complex has one of the structures:

In some embodiments, the cobalt coordination complex can be a polymer,e.g., a polymer containing a plurality of monomer units, each based upona cobalt coordination complex, wherein covalent bonds (e.g., sigmabonds) have been formed between atoms (e.g., carbon atoms) in thechelating ligand of one coordination complex monomer unit and atoms(e.g., carbon atoms) in the chelating ligand of additional monomerunits. In some embodiments, the polymer is provided byelectropolymerizing a solution of a cobalt coordination complex (e.g.,cobalt salen or a derivative thereof).

Any suitable gold-containing structure can be used, so long as at leasta portion of the surface of the structure comprises gold. Thus, the goldcan be gold in bulk form, e.g., microporous gold or any bulk gold. Thegold can be polycrystalline or have a single or dominantcrystallographic orientation (i.e., Au(100) or Au(111)). The gold can bea gold layer or film present on a surface (e.g., of an air or otherelectrode). For instance, the gold can be coated (e.g., via sputteringor any other suitable technique) onto a support material comprisinganother element or elements. In some embodiments, the support materialis carbon or another material that is present, for example, on a surfaceof an air electrode and/or in a fuel cell. Suitable carbon supportmaterials include, but are not limited to, synthetic graphite, naturalgraphite, amorphous carbon, hard carbon, soft carbon, acetylene black,mesocarbon microbeads, carbon black, Ketjen black, mesoporous carbon,porous carbon matrix, carbon nanotubes, carbon nanofibers, carbonpapers, carbon cloths, and graphene.

The gold can be present as particles, which can optionally include asupport material in addition to the gold, so long as the particlecomprises a gold surface, and which can have any suitable shape orshapes, including, but not limited to, spherical, disk-shaped,cone-shaped, cylindrical, pyramidal, prism-shaped, cube, cuboid, and thelike. The particles can be regular or irregular. In some embodiments,the particles are micro- or nanoparticles. In some embodiments, thenanoparticles have an average diameter that is about 100 nm or less,about 50 nm or less, or about 20 nm or less (e.g., about 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or about 20 nm).

In some embodiments, the gold nanoparticles have a gold surface andfurther comprise a support material, such as a carbon support material.The carbon support material can be, but is not limited to, carbon black,or one of the other carbon materials typically used in air electrodes.In some embodiments, the gold nanoparticles comprise between about 1weight % and about 30 weight % gold (e.g., about 1, 2, 4, 6, 8, 10, 12,14, 16, 18, 20, 22, 24, 26, 28, and about 30 weight % gold).

The cobalt complex can be present in a solution (e.g., an electrolytesolution) that is in contact with the gold or be present as a solid,e.g., immobilized non-covalently on a gold surface. In some embodiments,the cobalt complex is present as a thin film produced by drying asolution of the cobalt complex on a gold surface. In some embodiments,the cobalt complex is polymerized (e.g., electropolymerized) on a goldsurface.

In some embodiments, the catalyst composition comprises a weight ratioof gold to cobalt coordination complex of between about 2:1 to about1:30. In some embodiments, the weight ratio of gold to cobaltcoordination complex is between about 1:5 and about 1:15 (e.g., 1:5,1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, and 1:15). In someembodiments, the weight ratio of gold to cobalt coordination complex isabout 1:10.

In some embodiments, the presently disclosed subject matter provides amethod of preparing a catalyst for reversible, two electron oxygenreduction, wherein the method comprises:

(a) providing a solution comprising a cobalt coordination complex,wherein the cobalt coordination complex comprises a cobalt ion chelatedby a tetradentate organic chelating ligand;

(b) providing a structure comprising a gold surface; and

(c) contacting the structure with the solution from step (a).

In some embodiments, the tetradentate organic chelating ligand isN,N′-bis(salicylidene)ethylenediamine or a derivative thereof.

In some embodiments of the above-mentioned methods, the method furthercomprises: (d) electropolymerizing the cobalt coordination complex toform an electropolymerized film on the surface of the structure (i.e.,over a portion of the gold surface). Alternatively, following step (c),the solution of step (a) can be dried to form a film of non-polymerizedcoordination complex.

In some embodiments, the solution comprising the cobalt coordinationcomplex is prepared using an organic solvent, such as, but not limitedto, acetonitrile (ACN), dimethyl sulfoxide (DMSO), dimethyl formamide(DMF), N-methyl 2-pyrrolidone (NMP), and dimethyoxyethane (DME) oranother aprotic solvent. In some embodiments, the solution furthercomprises a salt, such as, but not limited to, tetrabutylammoniumbromide (TBAB) or tetrabutylammonium tetrafluoroborate.

In some embodiments, the cobalt coordination complex can be any complexhaving a structure of Formula (I) as described hereinabove or a polymerthereof. In some embodiments, the cobalt coordination complex isselected from cobalt salen, cobalt salophen, and a thiophene modifiedcobalt salen. In some embodiments, the structure comprising a goldsurface is a nanoparticle, e.g., a gold nanoparticle or a gold/carbonnanoparticle. In some embodiments, the structure is an electrode, suchas an air electrode, wherein the gold is present as a layer or coatingon a surface of the air electrode.

IV. Electrochemical Cells, Cell Components, and Systems

In some embodiments, the presently disclosed subject matter provides acomponent of an electrochemical cell that comprises a catalystcomprising gold and a cobalt coordination complex (e.g., cobalt salen ora derivative and/or polymer thereof). For example, the cell componentcan be an electrolyte comprising the catalyst, an electrode (e.g., agold or an air electrode) comprising the catalyst, or a support, such asa membrane or particulate structure that comprises the catalyst and canbe inserted into an electrolyte solution or layered onto a surface of anelectrode.

In some embodiments, the presently subject matter provides an airelectrode that comprises the catalyst. Any suitable air electrode can beprovided and coated with a layer or layers comprising the catalyst. Airelectrodes can include a carbon material (e.g., a porous carbonmaterial) having a surface. In some embodiments, the surface can becoated with a thin layer of an inert material. The layer can becontinuous or discontinuous. In some embodiments, the inert materiallayer can have a thickness of between about 0.1 nm and about 100 nm. Insome embodiments, a layer of a porous material (e.g., a carbon paper orcarbon cloth) can be added on the surface (e.g., of the inert materialor of the other carbon materials of the air electrode) as a gasdiffusion layer.

A thin layer of a metal or of metal containing nanoparticles (e.g.,metal oxide or metal/carbon nanoparticles) can be added as anovercoating to any or all of the other above-described surface layers.For instance, the metal or metal containing nanoparticles can includethe gold component of the presently disclosed catalyst. Thenanoparticles, if present, can have an average particle size of betweenabout 1 and about 100 nm (or between about 1 nm and about 50 nm, orbetween about 1 nm and about 20 nm). The metal or metal nanoparticlescan then be coated with a film and/or polymer of the cobalt coordinationcomplex.

The carbon material can be any carbon material known for use in airelectrodes. Such materials include, but are not limited to, syntheticgraphite, natural graphite, amorphous carbon, hard carbon, soft carbon,acetylene black, mesocarbon microbeads, carbon black, Ketjen black,mesoporous carbon, porous carbon matrix, carbon nanotubes, carbonnanofibers, and graphene. By “inert material” is meant a material thatis stable in an oxygen containing atmosphere (i.e., the material doesnot react with oxygen under ambient conditions). The material is alsostable to any electrolyte to which the air electrode will be exposed.Suitable inert materials include, but are not limited to metal oxides,metal halides, metal oxyfluoride, metal phosphate, metal sulfate,non-metal oxides, and non-metal elements (e.g., silicon).

The air electrodes can include multiple thin layers of carbon material.They can also include binders to hold that carbon material together orto maintain contact of the carbon material to a current collector thatcan be provided as part of the air electrode. Suitable binders include,but are not limited to, poly(acrylonitrile), poly(vinylidene fluoride),polyvinyl alcohol, polyethylene, polystyrene, polyethylene oxide,polytetrafluoroethylene, polyimide, styrene butadiene rubber, carboxymethyl cellulose, gelatin, or a copolymer or blend of any of thesematerials.

The coated carbon materials can be prepared by a variety of depositionmethods known for the deposition of thin layers and/or for catalysts forair electrodes. Such methods include, but are not limited to, atomiclayer deposition (ALD), sputtering, ink-jet, spray, co-precipitation,chemical vapor deposition (CVD), physical vapor deposition (PVD), orelectron beam deposition (EBD). In some embodiments, a film of thecobalt coordination complex is provided over a layer of gold orgold-containing nanoparticles via electropolymerization.

In some embodiments, the air electrode can be provided with an airelectrode current collector for collecting the current of the airelectrode. In some embodiments, the air electrode current collector canhave a porous structure or a dense structure as long as it has suitableelectron conductivity. In some embodiments, in view of the desireddiffusion of air (oxygen), a porous structure can be used. Examples ofporous structures include, for instance, mesh structures wherestructural fibers are regularly arranged, a nonwoven fabric structurewhere the structural fibers are arranged at random, and athree-dimensional network structure having independent pores orconnected pores. Examples of materials for the air electrode currentcollector include, but are not limited to, metals, such as stainlesssteel, nickel, aluminum, iron, titanium, and copper; carbon materials,such as carbon fiber, carbon cloth, and carbon paper; and ceramicmaterials having high electron conductivity, such as titanium nitride.In some embodiments, the thickness of the current collector can bebetween about 10 and about 1000 microns. In some embodiments, the airelectrode can be included in a battery and the battery case can serve asthe air electrode current collector.

In some embodiments, the presently disclosed subject matter provides anelectrochemical cell that comprises a catalyst comprising gold and acobalt coordination complex (e.g., cobalt salen or a derivative and/orpolymer thereof). In some embodiments, the presently disclosed subjectmatter provides a fuel cell or a battery (e.g., a metal-air battery)comprising the catalyst.

In some embodiments, the presently disclosed subject matter provides anair battery comprising a negative electrode (e.g., a metal electrode,such as a lithium or zinc electrode), an air electrode, and at least oneelectrolyte disposed between the electrodes. In some embodiments, theair electrode can be an air electrode as described hereinabove. Anysuitable electrolyte or combination of electrolytes can be used. Forexample, the electrolyte(s) can be aprotic (i.e., organic), aqueous,and/or solid-state (e.g., gel or polymer). In some embodiments, thebattery can include at least an aqueous electrolyte that is in contactwith the air electrode. In some embodiments, the solid-state electrolytehas a thickness of between about 0.1 and about 10 μm, and in someembodiments, between about 3 and about 5 μm.

The negative electrode can include a negative electrode active materialcapable of releasing and storing metal ions (conducting ions). Thenegative electrode can be provided with a negative electrode currentcollector that collects a current of the negative electrode. Thenegative electrode active material is not particularly limited as longas it can release and store conducting ion species (e.g., metal ions).For example, in some embodiments, the negative electrode can be a metalelectrode comprising a material selected from the group including, butnot limited to, a metal, a metal alloy, a metal oxide, a metal sulfideand a metal nitride, which contain a metal ion that is a conducting ionspecies. In some embodiments, a carbon material can be used as anegative electrode active material. In some embodiments, a metal ormetal alloy is used as the negative electrode active material. Suitablemetals include, but are not limited to, lithium (Li), sodium (Na),potassium (K), magnesium (Mg), calcium (Ca), aluminum (Al), zinc (Zn)and iron (Fe). In some embodiments, the negative electrode is a lithiumor zinc electrode. More particularly, as a negative electrode activematerial of a lithium-air battery, for example, lithium metal; lithiumalloys such as lithium aluminum alloy, lithium tin alloy, lithium leadalloy, and lithium silicon alloy; lithium oxide; lithium compositeoxides such as lithium titanate; lithium sulfides such as lithium tinsulfide and lithium titanium sulfide; lithium nitrides such as lithiumcobalt nitride, lithium iron nitride and lithium manganese nitride canbe employed.

The negative electrode can contain at least a negative electrode activematerial. In some embodiments, it can also contain a binder for fixingthe negative electrode active material. For example, when a foil-likemetal or alloy is used as a negative electrode active material, anegative electrode can be formed into a form having the negativeelectrode active material alone. However, when a powdery negativeelectrode active material is used, a negative electrode can be formedinto a form that contains the negative electrode active material and abinder. Further, the negative electrode can contain a conductivematerial. The kind and the amount used of the binder and the conductivematerial can be as described above for the air electrode.

In some embodiments, the negative electrode can be provided with anegative electrode current collector. A material of the negativeelectrode current collector is not particularly limited as long as ithas conductivity. For example, copper, stainless steel, and nickel canbe used, among others. As a shape of the negative electrode currentcollector, for example, foil, plate, and mesh can be used. In someembodiments, a battery case can serve as a negative electrode currentcollector.

As an electrolytic solution, for example, a non-aqueous (i.e., organic)electrolytic solution that contains a support electrolyte salt and anorganic solvent can be used. The organic solvent is not particularlylimited. For example, the organic electrolyte can include, but is notlimited to, propylene carbonate (PC), ethylene carbonate (EC), vinylenecarbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC),diethyl carbonate (DEC), methylpropyl carbonate, isopropiomethylcarbonate, ethyl propionate, methyl propionate, γ-butyrolactone, ethylacetate, methyl acetate, tetrahydrofuran, 2-methyltetrahydrofuran,ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ACN,DMSO, diethoxyethane; DME, and tetraethylene glycol dimethyl ether(TEGDME).

Further, an ionic liquid can be used in or as an organic electrolyticsolution. Suitable ionic liquids include, for example, but are notlimited to, aliphatic quaternary ammonium salts such as N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide(TMPA-TFSA], N-methyl-N-propylpiperidiniumbis(trifluoromethane-sulfonyl)amide (PP13-TFSA),N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)-amide(P13-TFSA), N-methyl-N-butylpyrrolidinumbis(trifluoromethanesulfonyl)-amide (P14-TFSA), andN,N-diethyl-N-methyl-N-(2-methoxyethyl)ammoniumbis(trifluoromethane-sulfonyl)amide (DEME-TFSA); and alkyl imidazoliumquaternary salts such as 1-methyl-3-ethyl imidazolium tetrafluoroborate(EMIBF₄), 1-methyl-3-ethyl imidazoliumbis(trifluoromethanesulfonyl)amide (EMITFSA), 1-allyl-3-ethylimidazolium bromide (AEImBr), 1-allyl-3-ethyl imidazoliumtetrafluoroborate (AEImBF₄), 1-allyl-3-ethyl imidazoliumbis(trifluoromethanesulfonyl)amide (AEImTFSA), 1,3-diallyl imidazoliumbromide (AAImBr), 1,3-diallyl imidazolium tetrafluoroborate (AAImBF₄),and 1,3-diallyl imidazolium bis(trifluoro-methanesulfonyl)amide(AAImTFSA). In some embodiments, the organic electrolyte comprises AcN,DMSO, DME, PP13-TFSA, P13-TFSA, P14-TFSA, TMPA-TFSA and/or DEME-TFSA.

Any support electrolyte salt is acceptable as long as it has solubilityin a electrolyte solvent and develops desired metal ion conductivity. Insome embodiments, a metal salt that contains a metal ion that is desiredto be conducted can be used. For example, in the case of a lithium-airbattery, a lithium salt can be used as a support electrolyte salt. As alithium salt, inorganic lithium salts such as LiPF₆, LiBF₄, LiClO₄,LiAsF₆, LiOH, LiCl, LiNO₃, and Li₂SO₄ can be used. Further, organiclithium salts such as CH₃CO₂Li, lithium bisoxalate borate (LiBOB),LiN(CF₃SO₂)₂(i.e., LiTFSA), LiN(C₂F₅SO₂) (i.e., LiBETA), andLiN(CF₃SO₂)(C₄F₉SO₂) can be used. In a nonaqueous electrolytic solution,a concentration of the support electrolyte salt can be set, though isnot particularly limited, in the range of 0.5 M to 3 M, for example.

In some embodiments, the electrolyte is an aqueous electrolyte andcomprises water. The aqueous electrolyte can also include a supportelectrolyte salt. In some embodiments, the aqueous electrolyte isalkaline. In some embodiments, the aqueous electrolyte has a pH of about9 or higher or about 12 or higher (e.g., between 9 and 14 or between 12and 14). In some embodiments, the pH is about 13. The pH of the aqueouselectrolyte can be adjusted, for example, by the addition of a suitablemetal hydroxide, e.g., KOH, NaOH, LiOH, etc. The aqueous electrolyte canalso include other support salts that have solubility in water and thatcan develop desired ionic conductivity. A metal salt that contains ametal ion that is desired to be conducted can be used. For example, inthe case of the lithium-air battery, for example, lithium salts such asLiCl, LiNO₃, Li₂SO₄, and CH₃COOLi can be used.

The electrolytic solution can be incorporated into a battery in a statewhere it is impregnated in a separator that has an insulating propertythat can ensure an insulating property between an air electrode and anegative electrode, and a porous structure that can retain theelectrolytic solution. As the materials of the separator, for example,insulating resins such as polyolefins including polyethylene andpolypropylene and glasses can be cited. Further, as a porous structureof the separator, for example, a mesh structure where structural fibersare regularly arranged, a nonwoven fabric structure where structuralfibers are arranged at random, and a three-dimensional network structurehaving independent pores or coupling holes can be cited. A thickness ofthe separator can be, for example, about 10 to 500 microns.

Electrolyte gels can be obtained by gelating the electrolytic solutionsdescribed above. For example, as a method of gelating a nonaqueouselectrolytic solution, a, polymer such as polyethylene oxide (PEO),polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) or polymethylmethacrylate (PMMA) can be added to a nonaqueous electrolyte solution.An electrolyte gel can be formed, for example, in such a manner that,after the polymer and the electrolytic solution, which were describedabove are mixed, the mixture is coated by casting on a base material anddried, the dried mixture is peeled off the base material, and cut intopieces as required.

Solid electrolytes can be appropriately selected in accordance with aconductive metal ion without particular limitation. For example, in thecase of a lithium-air battery, LISICON (i.e., lithium super ionicconductor) oxides represented by Li_(a)X_(b)Y_(c)P_(d)O_(e) (wherein Xrepresents at least one kind selected from the group of B, Al, Ga, In,C, Si, Ge, Sn, Sb and Se; Y represents at least one kind selected fromthe group of Ti, Zr, Ge, In, Ga, Sn and Al; and a to e satisfyrelationships of 0.5≤a≤5.0, 0≤b≤2.98, 0.5≤c<3.0, 0.02<d.≤3.0,2.0<b+d<4.0, and 3.0<e≤12.0); perovskite oxides such asLi_(x)La_(1-x)TiO₃; LISICON oxides such as Li₄XO₄—Li₃YO₄ (wherein Xrepresents at least one kind selected from Si, Ge and Ti, and Yrepresents at least one kind selected from P, As and V) andLi₃DO₃-Li₃YO₄ (wherein D represents B, Y represents at least one kindselected from P, As and V); and garnet oxides of Li—La—Zr—O based oxidessuch as Li₇La₃Zr₂O₁₂ can be used. The solid electrolyte can be molded,for example, by rolling, or by preparing a slurry by mixing with asolvent, by coating, and by drying.

The presently disclosed batteries can also have other constituentmembers other than the air electrode, electrolyte(s), and negativeelectrode, which were described above. Typically, the battery can have abattery case for housing an air electrode, a negative electrode and anelectrolyte. The shape of the battery case is not particularly limited.For example, a coin shape, a flat plate shape, a cylindrical shape, anda laminate shape can be used. The battery case can be either an openatmosphere type or a hermetically sealed type as long as it can feedoxygen to the air electrode. The open atmosphere type battery case has astructure where at least an air electrode is capable of sufficientlycoming into contact with atmosphere. For instance, the case can includeoxygen-intake holes that communicate with the air electrode. Theoxygen-intake holes can include an oxygen transmitting membrane that canselectively transmit oxygen. In some embodiments, a polysiloxane-basedmembrane can be used. On the other hand, in a hermetically sealed typebattery case, an inlet pipe and an outlet pipe of oxygen (air) can beprovided.

In some embodiments, the presently disclosed subject matter provides, asan exemplary electrochemical cell, a lithium air battery that comprisesa lithium electrode, a porous support comprising an organic electrolyte;a solid state electrolyte, an aqueous electrolyte, a layer comprisingthe catalyst, and an air electrode. One possible configuration of such abattery is shown in FIG. 11. As shown in FIG. 11, the battery caninclude a protected lithium electrode that is in contact with the poroussupport material. The porous support material comprising an organicelectrolyte is disposed between the anode and the solid stateelectrolyte. The organic electrolyte can comprise, for instance, DMSOand LiClO₄; but can also comprise another organic electrolyte and/orsalt as described hereinabove. In some embodiments, the solid stateelectrolyte is a LISICON material.

In some embodiments, the porous support can have a thickness of about200 microns. In some embodiments, the solid state electrolyte can have athickness of between about 3 microns and about 5 microns. However, thesupport material and solid state electrolytes can have any suitable ordesired thickness. In some embodiments, the support material and solidstate electrolyte can be as thin as possible while still havingmechanical stability.

Referring again to FIG. 11, the solid state electrolyte in turn isdisposed between the porous support material and the aqueous electrolyte(e.g., an alkaline aqueous electrolyte). Accordingly; lithium from thelithium electrode can be reduced to lithium anions and be delivered tothe aqueous electrolyte though the porous support and the solid stateelectrolyte, while the lithium electrode is protected from water in theaqueous electrolyte.

The aqueous electrolyte is also in contact with a catalyst layer that isin contact with a gas diffusion layer (GDL) of an air electrode. The GDLis a porous structure (e.g., a carbon cloth or paper) that deliversoxygen (O₂) from the air electrode to the catalyst layer, where it isreduced to hydroxide. In some embodiments, the aqueous electrolyte has apH of about 13 or more. In some embodiments, the aqueous electrolyte canfurther comprise a peroxide stabilizer, such as sodium silicate or atransition metal ion complexing or chelating agent, such as EDTA or DTPAor another aminopolycarboxylic acid, or a phosphonate. As shown in FIG.11, the lithium air battery can have a flowing electrolyte, in that thebattery can include an inlet and an outlet for the aqueous electrolyte,so that it can flow through the cell, transferring lithium peroxideformed during discharge out of the cell, e.g., where it can be stored ina solid phase. Thus, in some embodiments, the cell is connected (i.e.,via the inlet and outlets) to one or more storage and/or separationunits, containers or tanks where the lithium peroxide can be separatedfrom the aqueous electrolyte solution and/or stored. The cell can alsoinclude current collectors associated with the lithium anode and the airelectrode and a circuit to carry electric current. In some embodiments,the one or more storage and/or separation units is/are configured tostore discharge product, such as but not limited to air electrodedischarge product, as a condensed liquid or solid. In some embodiments,the storage unit or tank is used to preserve the air electrode dischargeproduct when the battery discharges. For a better preservation purpose,a separation process can be employed to separate the discharge productfrom the bulk electrolyte phase to preserve the discharge product in asolid phase or condensed liquid phase.

FIGS. 10A and 10B further illustrate an embodiment of the presentlydisclosed subject matter that comprises a flowing electrolyte lithiumair battery. FIG. 10A shows the lithium air battery with a flowingaqueous electrolyte during discharge. The battery includes a lithiumanode, a lithium ion conducting solid membrane, an aqueous electrolyte,a catalyst layer and an air electrode. During discharge, lithiumperoxide (Li₂O₂) is formed in the aqueous electrolyte. From an outlet inthe cell, the electrolyte can flow from the cell to a separation tank,where, for example, the electrolyte can be cooled to cause precipitationof the lithium peroxide. The remaining aqueous electrolyte can leave theseparation tank via an outlet and be contacted with concentrated LiOHand the resulting alkaline electrolyte circulated back into the cell viaa cell inlet. The electrolyte can still include some Li₂O₂ due toincomplete separation in the separation/storage unit. FIG. 10B shows thesame lithium air battery during charging. In some embodiments, the oneor more storage and/or separation units is/are configured to storedischarge product, such as but not limited to air electrode dischargeproduct, as a condensed liquid or solid. In some embodiments, thestorage unit or tank is used to preserve the air electrode dischargeproduct when the battery discharges. For a better preservation purpose,a separation process can be employed to separate the discharge productfrom the bulk electrolyte phase to preserve the discharge product in asolid phase or condensed liquid phase.

Energy density for the battery can be calculated from the energy thatcan be stored divided by the mass of lithium metal and how much water isneeded to store the lithium peroxide in the separation/storage unit.Assuming that 1 equivalent of H₂O per lithium ion is needed to dissolveand precipitate out one molecule of Li₂O₂ so that it can be stored, theenergy density can be 3277 watt-hours (Wh)/kg. If 2 equivalents of H₂Ois needed, the energy density can be 1903 Wh/kg. If 3 or 4 equivalentsof H₂O is needed, the energy density can be 1341 or 1035 Wh/kg,respectively.

Referring now to FIG. 12, a safety procedure is provided to control thebattery explosion hazard to minimum when receive outside collisionaccident. When receiving outside collision accident, the thin solid Li+conducting electrolyte can be cracked. If lithium metal gets in touchwith aqueous electrolyte, it will generate a lot of hydrogen, and thismight lead to potential explosion. The special safety procedure shownabove can minimize this hazard. When receiving outside accident, theelectrolyte inlet starts sending inert gases into the cell rather thannormal aqueous electrolyte. At the same time, the electrolyte outlet ofthe cell still purging aqueous electrolyte out of the cell to minimizethe amount of water that can get in touch with lithium metal. Andincoming oxygen is also cut from the cathode side, and the rest of theoxygen is moved out of the cell with nitrogen. This whole process canreduce the potential explosion of aqueous lithium-air battery.

Thus, continuing with reference to FIG. 12, in accordance with someembodiments, a cell is configured to provide one or more of thefollowing in the event of an accident: (i) the electrolyte inlet sendsan inert gas into the cell rather than aqueous electrolyte; (ii) theelectrolyte outlet of the cell purges aqueous electrolyte out of thecell to minimize the amount of water that can get in touch with metal,optionally lithium metal; and (iii) incoming oxygen is cut from thecathode side and the rest of the oxygen is moved out of the cell withnitrogen. Optionally (i), (ii) and/or (iii) occur at the same time.

In some embodiments, the presently disclosed subject matter provides azinc air battery. In some embodiments, the zinc air battery comprises anair electrode as described herein above, a zinc electrode, and one ormore electrolytes. In some embodiments, the zinc air battery comprises azinc electrode (e.g., a porous zinc electrode), an anionic exchangemembrane, and an air electrode associated with a layer comprising thecatalyst. For instance, as shown in FIG. 9A, the anion exchange membranecan be in contact with a surface of the zinc electrode and with thelayer comprising the catalyst, and the layer comprising the catalyst canbe in contact with both the anion exchange membrane and the airelectrode and/or a gas diffusion layer associated with the airelectrode. Oxygen can be transported from the air electrode to thecatalyst layer and reduced to peroxide. The battery can further includean outlet to transport aqueous hydrogen peroxide solution out of thebattery (e.g., from an outlet in the air electrode) for separationand/or storage. Thus, in some embodiments, for example, the airelectrode is in flow communication with a storage tank containing anaqueous solution of hydrogen peroxide. The other discharge product,i.e., zincate, formed from zinc oxide and alkaline can form a solidcomplex that can be preserved, through use of the membrane, on thesurface of the zinc electrode and/or replace the zinc electrode activematerial.

In some embodiments, the aqueous solution of hydrogen peroxide comprisesa peroxide stabilizer. In some embodiments, the peroxide stabilizer isoptionally sodium silicate or a transition metal ion complexing orchelating agent. In some embodiments, the peroxide stabilizer isethylenediamine tetraacetic acid (EDTA), diethylenetriaminepentaaceticacid (DTPA) or a phosphonate. In some embodiments, the at least oneoutlet and the at least one inlet are connected to one or more storageand/or separation tanks. In some embodiments, the one or more storageand/or separation units is/are configured to store discharge product,such as but not limited to air electrode discharge product, as acondensed liquid or solid. In some embodiments, the storage unit or tankis used to preserve the air electrode discharge product when the batterydischarges. For a better preservation purpose, a separation process canbe employed to separate the discharge product from the bulk electrolytephase to preserve the discharge product in a solid phase or condensedliquid phase. In some embodiments, the hydrogen peroxide is transferredout of the cell for storage, purification, and/or stabilization.

In some embodiments, the presently disclosed subject matter provides anelectrochemical cell that can be used in the fuel cell or electrolysermode, and that comprises the presently disclosed catalyst. One mode(i.e., the fuel cell) inputs hydrogen, while the other inputselectricity. The catalyst can be part of an air electrode, as describedhereinabove. In some embodiments, the presently disclosed subject mattercan provide a fuel cell comprising the presently disclosed catalyst(i.e., as part of a gas diffusion electrode which, in some embodiments,can have a modified Au mesh or Au nanoparticles catalyst prepared viaelectropolymerization of a cobalt coordination complex of Formula (I))and that can further comprise a hydrogen source and be useful in theproduction of hydrogen peroxide. See FIG. 8A, right-hand side. Anaqueous alkaline electrolyte can flow through the cell. Peroxidedissolved in the electrolyte can be carried out of the cell, asindicated by the thick arrow. The reactions at the anode and cathode canbe as follows:Anode: H₂+2OH⁻→2e ⁻+2H₂OCathode: O₂+2H₂O+2e ⁻→2OH⁻+H₂O₂

The left-hand side of FIG. 8A shows an electrolyser comprising thepresently disclosed catalyst (i.e., as part of a gas diffusion electrodewhich, in some embodiments, can have a modified Au mesh catalystprepared via electropolymerization of a cobalt coordination complex ofFormula (I)). It further uses a nickel mesh as an oxygen evolutioncatalyst (i.e., as part of the anode) and an anion exchange membranebetween the anode and cathode. Hydrogen peroxide can be removed from thecell via an outlet in the cathode (e.g., as an aqueous solution). Thereactions at the anode and cathode can be as follows:Anode: 4OH⁻→4e ⁻+2H₂O+O₂Cathode: O₂+4H₂O+2e ⁻→2OH⁻+H₂O₂

V. Methods of Performing Two Electron, Reversible Oxygen Reduction

In some embodiments, the presently disclosed subject matter provides amethod for performing a two electron reduction of oxygen, wherein themethod comprises contacting oxygen with water in the presence of acatalyst comprising gold and a cobalt coordination complex. In someembodiments, the reduction is reversible or near-reversible, bothchemically and electrochemically. In some embodiments, the cobaltcoordination complex comprises cobalt chelated to a tetradentate organicchelating ligand. In some embodiments, the organic chelating ligand isN,N′-bis(salicylidene)ethylenediamine or a derivative or polymerthereof. In some embodiments, the coordination complex has a structureof Formula (I), above, or a polymer thereof. In some embodiments, thecobalt coordination complex is cobalt salen, a thiophene modified cobaltsalen, or cobalt salophen.

In some embodiments, the weight ratio of gold to cobalt coordinationcomplex is any suitable ratio, such as but not limited to 1:5, 1:6, 1:7,1:8, 1:9, and 1:10. In some embodiments, the cobalt coordination complexis in an aqueous solution in contact with the gold. In some embodiments,the cobalt coordination complex is present in a coating covering asurface of a gold structure, such as, but not limited to, bulk gold, agold film or gold-containing nanoparticles. In some embodiments, thecobalt coordination complex is polymerized (e.g., electropolymerized)over a gold surface. In some embodiments, the catalyst is present in anelectrochemical cell, e.g., as part of an air electrode.

In some embodiments, the contacting of the oxygen and the catalyst isperformed in an alkaline aqueous solution that is saturated with oxygen.The aqueous solution can also include one or more electrolyte supportsalts. The alkaline solution can have a pH of at least 9 or of betweenabout 9 and about 14. In some embodiments, the pH is between about 12and about 14 (e.g., about 12, 12.5, 13, 13.5 or 14). In someembodiments, the pH is about 13. In some embodiments, the alkalinesolution comprises a metal hydroxide, such as KOH, NaOH, or LiOH. Insome embodiments, the metal hydroxide concentration is about 0.1 M ormore. In some embodiments, the metal hydroxide concentration is about1.0 M or about 3.0 M. Thus, in some embodiments, the contacting isperformed in an alkaline environment that is also saturated with oxygen.

In some embodiments, the contacting is further performed in the presenceof a peroxide stabilizer, such as sodium silicate or a transition metalion chelating group. In some embodiments, the peroxide stabilizer is anaminopolycarboxylic acid, such as, ethylenediaminetetraacetic acid(EDTA) or diethylenetriaminepentaacetic acid (DTPA), or a phosphonate.

In some embodiments, the contacting is performed at a temperature ofbetween about −20° C. and about 80° C. In some embodiments, thecontacting is performed at a temperature of between 20° C. and about 80°C. In some embodiments, the contacting is performed at a temperature ofbetween about 20° C. and about 50° C. (e.g., at about 20, 25, 30, 35,40, 45, or about 50° C.). In some embodiments, the contacting isperformed at a temperature of between about 20° C. and about 35° C.

In some embodiments, the reduction produces hydrogen peroxide. Thus, insome embodiments, the presently disclosed subject matter provides amethod comprising contacting oxygen with an aqueous solution (e.g., analkaline aqueous solution) in the presence of a catalyst comprising goldand a cobalt coordination complex to provide hydrogen peroxide. In someembodiments, the catalyst comprises gold and a cobalt coordinationcomplex of Formula (I) or a polymer thereof. In some embodiments, thecobalt coordination complex is cobalt salen, a thiophene modified cobaltsalen, or cobalt salophen.

In some embodiments, the catalyst is present in a fuel cell comprising asource of oxygen. In some embodiments, the catalyst is present in anelectrochemical cell further comprising a zinc electrode, an airelectrode, and an anion exchange membrane, wherein the anion exchangemembrane is present between the zinc electrode and the air electrode,the catalyst is contact with the air electrode and/or a gas diffusionlayer of the air electrode, and optionally wherein the zinc electrode isporous.

In some embodiments, the catalyst is present in an electrochemical cellfurther comprising a hydrogen electrode or a oxygen evolution electrode.In some embodiments, between the two electrodes, an alkaline electrolyteis used, which optionally can be an anion exchange membrane or anaqueous alkaline electrolyte. In some embodiments, the hydrogen peroxidecan be stored, purified, and/or stabilized.

EXAMPLES

The following Examples have been included to provide guidance to one ofordinary skill in the art for practicing representative embodiments ofthe presently disclosed subject matter. In light of the presentdisclosure and the general level of skill in the art, those of skill canappreciate that the following Examples are intended to be exemplary onlyand that numerous changes, modifications, and alterations can beemployed without departing from the scope of the presently disclosedsubject matter.

Example 1 Materials and Methods

Electrode Preparation:

Three types of 5 mm diameter disk electrodes were purchased from PineResearch Instrumentation (Durham, N.C., United States of America) foruse in the studies described below: polycrystalline gold (Pine ResearchInstrumentation part number AFE2A050AU for room temperature; part numberAFE5TO50AUHT for high temperature); glassy carbon (part numberAFE2A050GC) for room temperature; and polycrystalline platinum (partnumber AFE5TO50PTHT) for high temperature. The polycrystalline metalelectrodes were mechanically cleaned by polishing with 5 μm aluminapowder and then with 0.05 μm powder (Beuhler, Dusseldorf, Germany).Then, the mechanically cleaned metal electrodes were electrochemicallycleaned by CV in a 2.2 V scan range with 50 mV/s scan rate in 0.1 MHClO₄ acid electrolyte to remove covalently bonded chemical. Theelectrodes were also scanned in the normal electrochemical window forseveral cycles until the standard CV features were stable. The glassycarbon disk was mechanically cleaned, but not electrochemically cleaned.

In addition, two RRDE electrodes (also from Pine ResearchInstrumentation, Durham, N.C., United States of America) were used: agold disk RRDE with a 5 mm diameter gold insert disk (part numberAFED050P040AU) and a platinum ring (part number AFE6R1PT; 6.5 mm innerdiameter 7.5 mm outer diameter; collection efficiency 25%); and a glassycarbon disk RRDE (part number AFE7R9GCPT), having a glassy carbon diskdiameter of 5.61 mm, a ring inner diameter of 6.25 mm; an outer diameterof 7.92 mm, and a collection efficiency of 37%. Both RRDE electrodeswere cleaned as described for the disk electrodes, with the diskelectrode and ring electrodes going through the electrochemical cleaningprocedure separately.

Modified Electrode Preparation:

Three kinds of modified electrodes were prepared: a cobalt salenmodified polycrystalline gold electrode, a Pt/C catalyst modified glassycarbon electrode; and an Au/C catalyst modified glassy carbon electrode.More particularly, for some studies, experiments were conducted in acobalt salen-containing electrolyte. However, a thin film modificationmethod was adopted to make a quasi-stable cobalt salen modifiedelectrode. According to this method, a clean polycrystalline gold diskelectrode (polycrystalline gold RDE or RRDE) was prepared and then 15 μlof a 2 mM cobalt salen solution was deposited on the surface of the goldelectrode. The electrode was dried for 30 minutes and a thin layer filmwas formed of the surface of the electrode.

The Pt/C catalyst and Au/C modified glassy carbon electrode wereprepared by depositing 12 μp ink on the surface of a glassy carbonelectrode. The ink was prepared using 5% NAFION™ solution (SigmaAldrich, St. Louis, Mo., United States of America) in a 30/70 ionomer tocatalyst ratio in methanol. The 30% Pt/C catalyst (BASF, Florham Park,N.J., United States of America) had nano Pt particles of a diameter ofabout 2 nm. The Au/C catalyst was prepared by two different methods, theTurkevich method and magnetron sputtering, using two differentnanoparticle sizes, referred to as Au/C-a and Au/C-b.

According to the first method, an aqueous solution of chloroauric acid(H[AuCl₄], 88.4 mg in 400 ml H₂O) was heated to 70° C. Then trisodiumcitrate solution (212 mg in 4 ml H₂O) was added to start to reduce thechloroauric acid and the mixture was stirred for 3 hours at 70° C. Afterthe solution was cooled, an ethanol dispersion of carbon black (XC-72,Cabot Corporation, Boston, Mass., United States of America) was added,the mixture was stirred for 20 minutes and then centrifuged. The solidswere washed with ethanol and water and vacuum dried to provide a finepowder-like catalyst. The gold-to-carbon ratio was calculated from theratio of starting materials to be about 30% by weight. The size of thenanoparticles was determined using the Scherrer equation and X-raydiffraction (XRD) data to be about 18 nm.

Alternatively, the Au/C catalyst was prepared by magnetron sputtering byfirst placing the carbon black and two TEFLON® coated stir bars in a setof stainless steel cups attached to a vacuum compatible motor. A highpurity gold target (99.99%) was sputtered at an applied power of 14 Wfor 160 minutes, by direct current magnetron sputtering in an argon (Ar)atmosphere onto the continuously rotating carbon black. Gold loading wasdetermined using an inductively coupled plasma (ICP) optical emissionspectrometer to be 4.77%, and average particle size (determined fromscanning transmission electron microscopy (STEM) data) was about 1.8 nm.

Electrochemical Measurements:

Electrochemical measurements were conducted in a standardthree-compartment electrochemical cell sealed from the ambientenvironment. For RDE studies, a modulated speed rotator (Pine ResearchInstrumentation, Durham, N.C., United States of America) was used. Apotentiostat (SP-200, Bio-Logic SAS, Claix, France) was used formeasurements for three-electrode cell systems and a multi-channelpotentiostat (VMP3, Bio-Logic SAS, Claix, France) was used for RRDE fourelectrode systems. A gold wire counter electrode was separated from theworking electrode by a porous glass frit. A Hg/HgO reference electrodeor a double function saturated Ag/AgCl reference electrode was used formeasurements in alkaline electrolyte and a saturated Hg/HgSO₄ referenceelectrode was used for measurements in acidic electrolytes. In someinstances, the reference electrode was calibrated to the reversiblehydrogen electrode (RHE) by sparging the cell with H₂ and measuring theopen circuit potential at a Pt working electrode. For RDE experiments, aTEFLON® plug with a bearing was used to cap the electrode entrance andfit snugly around the shaft of a rotating electrode. Either O₂ or Aratmospheres were maintained with a slight positive pressure generated bya water or oil bubbler on the gas outlet from the cell.

For alkaline electrolyte electrochemical measurements, alkaline solutionwas made with potassium hydroxide, lithium hydroxide (both from SigmaAldrich, St. Louis, Mo., United States of America) and Mili-Q water. Aglass electrochemical cell (part number RRPG085, Pine. ResearchInstrumentation, Durham, N.C., United States of America) with a waterjacket was used for temperature controlling experiments, and a heatedwater bath circulator was used to control the temperature. EDTA(Sigma-Aldrich, St. Louis, Mo., United States of America) was used as aperoxide stabilizer. For organic electrolyte electrochemicalmeasurements, the glass electrochemical cell was dried in the oven toremove water and an organic electrolyte was prepared with acetonitrile(J.T. BAKER® brand, Avantor Performance Materials, Center Valley, Pa.,United States of America) and tetrabutylammonium bromide (TBAB,Sigma-Aldrich, St. Louis, Mo., United States of America). Cobalt salen(Sigma-Aldrich, St. Louis, Mo., United States of America) was used inboth aqueous and organic electrolyte.

Before every electrochemical measurement, the potential was held atopen-circuit voltage (OCV) for 30 minutes. For the cyclic voltammetry(CV) studies, after the OCV potential holding, the potential was scannedfrom the OCV potential to the lower potential and then back again.

Tafel plots were prepared by plotting the log (current density) versuspotential from a related RDE study kinetic region. The HCD (high currentdensity) range tafel slope was calculated by selecting data points from0.1 mA/cm² to 1 mA/cm². The LCD (low current density) range tafel slopewas calculated by selecting data points from 0.01 mA/cm² to 0.1 mA/cm².

Example 2 Oxygen Reduction on Cobalt Salen Modified Gold Electrode

A polycrystalline Au electrode was mechanically and electrochemicallycleaned as described above in Example 1. It was then tested in a purealkaline electrolyte solution (1 M KOH saturated with oxygen). Then 10μl of a 1 M KOH+2 mM cobalt salen solution was deposited on theelectrode surface and dried for 30 minutes, forming a thin film. Thethin film-modified electrode was placed back into the alkalineelectrolyte solution. FIG. 1 shows a comparison of the ORR CV of theelectrode before and after modification with the cobalt salen thin film.

FIG. 1 shows CVs indicating that the modified Au electrode (solid lineCV) processes a two electron, reversible oxygen reduction reaction. Inthe solid line CV, oxygen reduction occurs in the cathodic scan andoxygen evolution in the anodic scan. For the unmodified Au electrode(dotted line), there is oxygen reduction in the cathodic scan, but nooxygen evolution in the anodic scan.

FIG. 2 shows tafel plots of data from RDE experiment data tocharacterize the kinetics of the electrochemical reactions of themodified and unmodified Au electrode in the alkaline, oxygen-saturatedelectrolyte. From the plots in FIG. 2, it appears that the slope of theplot related to the modified electrode has a lower value (56 mV/decade(dec) at HCD range and 36 mV/dec at LCD range), indicating that oxygenreduction is faster with the modified electrode.

It has previously been suggested that the rate limiting step of ORR onbare gold surfaces is the first electron transfer (see Zurilla, R. W.,Electrochim. Acta (1978), 125, 1103; and Erickson, H., et al.,Electrochim. Acta (2009), 54, 7483-7489), which can be related to thechemisorption of oxygen onto the gold surface: O₂+e⁻→O₂ ⁻(ads). Thus,without being limited to any one theory, the rate improvement with thecobalt salen film-modified Au electrode is believed to be related toenhanced oxygen chemisorption on the Au surface that is associated withreversible bonding of oxygen to cobalt salen. Since it is furtherbelieved that the cobalt salen is interacting with the gold surface viaphysisorption (i.e., adsorbtion via weak Van der Waals forces), ratherthan being more strongly associated, such as by being chemrisorbed onthe Au, again without being bound to any one theory, it is suggestedthat the cobalt salen is within the Outer Helmholtz Layer (OHL) of thegold surface. When an oxygen molecule bonds to the cobalt salen, it canhave a lower electron density and be a better electron accepter comparedto normal oxygen. Based on calculations, the oxygen molecule length whenbonded to cobalt salen complexes is in the range of 1.28 to 1.29angstroms, which is the same as the molecule length when oxygen ischemisorbed on Au. Thus, it is possible that pre-interaction of O₂ withcobalt salen stretches the molecule, making it more readily chemisorbedon Au. See FIG. 3A. Alternatively, cobalt can become a reaction centerand finish the first electron reaction via a tunneling electron transferfrom the Au surface. See FIG. 3B.

Example 3 Effects of pH and Temperature

Temperature:

The catalytic activity of a gold electrode in an oxygen saturatedalkaline electrolyte containing cobalt salen (1 M KOH+2 mM cobalt salen)was measured at three different temperatures: 25° C., 50° C., and 70° C.The catalyst had much high reduction current density at 50° C. and 70°C. as compared to 25° C. This higher reduction current appears to becaused, at least in part, by the further reduction of hydroperoxide tohydroxide. The catalyst also had a steeper reduction curve at 70° C.than at 50° C., implying that the chemisorption of the hydroperoxide onthe Au surface was favored at high temperatures. In addition, an anodicpeak current density decreased with increasing temperature, which isbelieved to be due to less production of peroxide. Since the catalyticactivity of a pure polycrystalline gold electrode for peroxideproduction is also greatly changed by increasing temperature, thissimilar data for the gold/cobalt salen combination indirectly suggeststhat Au is mainly responsible for the reversible nature of the catalyticactivity of the catalyst (while the cobalt salen is believed to beresponsible for reactant centralization and reaction kinetics).

pH:

To determine the effects of pH on the catalytic activity of theAu/cobalt salen catalyst, a series of CV studies were performed using acobalt salen film-modified polycrystalline gold electrode in oxygensaturated alkaline electrolytes having different concentrations of KOH:0.01 M KOH, 0.1 M KOH, 1 M KOH, and 3 M KOH. Based on the shape of theCVs, it was determined that the catalyst worked well in 1 M KOH and 3 MKOH. For 0.1 M and 0.01 M KOH solutions, the reversibility started todeteriorate with lower KOH concentration. This observation could berelated to a change in the composition of the double layer near theelectrode surface when the alkaline concentration is low and to themechanism of peroxide oxidation being sensitive to the double layerstructure. Alternatively or additionally, this observation could berelated to changes in the decomposition rate of hydroperoxide ion whenthe hydroxyl radical concentration becomes lower.

Example 4 Effects of Peroxide Stabilizer

During the oxidation reduction process of the presently disclosedAu/cobalt salen systems, hydroperoxide can be produced byelectrochemical oxygen reduction and then chemically decomposes inalkaline environment. To reduce battery recharge overpotential and toincrease recharge speed, peroxide can be preserved. Since peroxideself-decomposition can be related to Fenton processes caused bytransition metal ions (see Croft et al., J. Chem. Soc. Perkin Trans.(1992) 2, 153-160), transition metal ion defunctionalizers can be usedas peroxide stabilizers. For instance, a commonly used compound issodium silicate, which can stabilized peroxide with or withouttransition metal ions. Transition metal complexing or chelating agents,such as DTPA and EDTA can also be used as peroxide stabilizers.

Accordingly, EDTA was selected as an exemplary electrolyte additive tostabilize peroxide, and CV of a cobalt salen film modifiedpolycrystalline gold electrode was performed in an oxygen saturatedelectrolyte containing 1 M KOH and 1 M EDTA. The ratio of peak reductioncurrent density over peak oxidation current density was 1:0.99, whichindicates that the peroxide produced in the oxygen reduction process isessentially all oxidized back. This also further indicates that for theAu/cobalt salen catalyst, the oxygen reduction process is a pure twoelectron reduction process, since the reduction product can be oxidizedwithout going to higher overpotential.

Example 5 Electropolymerization of Cobalt Salen

Since the electropolymerization potential of salen is out of the aqueouselectrochemical window, electropolymerization of cobalt salen wascarried out in an organic electrolyte environment. Thus, a solutioncomprising 2 nM cobalt salen and 50 mM TMBAF₄ was prepared inacetonitrile and electropolymerized on a polycrystalline gold electrode(scan rate 50 mV/sec.). For comparison, cobalt salen was alsoelectropolymerized on a high density pyrolytic graphite (BHPG)electrode. With the BHPG electrode, current-potential curves of theprogress of the oxidative electropolymerization of cobalt salen showedthat peaks at 1.35 V developed. In addition, a peak for Co(II/III) at0.4 V diminished while a peak for Co(I/II) at −1.2 V increased withrepeated scanning. A greenish film was formed. In a plot showing curvesfor 12 cycles of CV for the gold electrode, the Co(II/IIII) peak shiftedpositive and appeared sharper than in the curves forelectropolymerization on the BHPG electrode, indicating that Co²⁺oxidation was becoming more difficult. Without being bound to any onetheory, this shift could also suggest that surface reconstruction wasoccurring. Thus, electropolymerization of cobalt salen on gold appearsto not only provide a stable, insoluble film on the gold surface tostabilize the cobalt salen/gold combination, but could also provide astable gold surface reconstruction.

After electropolymerization, the modified electrode was tested in 1 MKOH electrolyte to assess effects of the polymerization on ORR. See FIG.4B. By comparing FIG. 4B to FIG. 4A (which shows CVs of free cobaltsalen on an Au electrode), it can be seen that after the cobalt salenhas been electropolymerized on the Au surface, the catalyst goes througha break-in (the first cycle showed relatively poorer reversibility), andthen displays better catalytic stability, than cobalt salen in solutionin contact with the Au.

Scanning electron microscopy was used to further inspect anelectropolymerized cobalt salen film on an Au sputtered gas diffusionelectrode (GDE). Using the same conditions as with the polycrystallinegold electrode, a non-homogenous film was formed. Elemental mappingindicated that the coverage of modification was good and that theelemental composition of the film was as expected.

Example 6 Thiophene-Modified Cobalt Salen

Thiophene-modified cobalt salen was prepared by a two step process.First, thiophene modified salicylaldehyde was synthesized by5-Bromosalicylaldehyde and thiophene-2-boronic acid by Suzuki reactionmethod. Then the target chemical was made by the condensation of thefirst step product and ethylenediamine.

Thiophene-modified cobalt salen was electropolymerized on apolycrystalline gold electrode as described above in Example 4 forcobalt salen to form a film. It was observed to be easier to polymerizethan an original cobalt salen electropolymerized film. FIG. 5 shows theCV of the thiophene-modified cobalt salen-modified gold electrode inalkaline electrolyte (1 M KOH saturated with oxygen).

Example 7 Production of Hydrogen Peroxide

FIG. 6 shows results from an RRDE experiment to determine the electrontransfer number of the presently disclosed catalysts using a RRDEelectrode comprising the catalyst deposited on the disk electrode. FIG.7A shows data from RDE studies of catalyst catalytic activity using acarbon catalyst (carbon black), a Pt/C catalyst (i.e., Pt on carbonblack), and a presently disclosed catalyst (Au/cobalt salen). FIG. 7Bshows the tafel plots for the catalysts (i.e., C, Aulcobalt salen, andPt/C) from FIG. 7A, which demonstrate the differences in kineticperformance of the catalysts.

The Au/cobalt salen catalyst promotes oxygen reduction with higherkinetics and onset potential than carbon, while still remaining over 90%selective for two electron oxygen reduction over a wide potential range.See FIGS. 6 and 7A. As seen in FIG. 6, the catalyst (i.e., Au/cobaltsalen, weight ratio about 1:10), has good catalyst activity and remainstwo electron reduction over a wide potential range. In FIG. 7A, theAulcobalt salen shows much higher reduction onsite potential then carbonand only a little lower then Pt/C. In addition, as shown in FIG. 7B, thekinetic performance of the Au/cobalt salen catalyst was comparable withPt, the most commonly used ORR catalyst in high performance fuel cellsystems, while the current density difference (see FIG. 7A) shows thedifference between the two electron ORR and four electron ORR of Pt.

The flow field design for high current density fuel cells has recentlybeen significantly advanced. However, this knowledge is not as useful ifthe fuel cell is built based on a low performance carbon ORR catalyst.Based on the presently disclosed high performance two electron ORRcatalyst, existing knowledge of fuel cell flow field design can beadopted to build better high performance and highly efficient fuel cellsto produce hydrogen peroxide. FIG. 8A shows the design of electrolyticcells for hydrogen peroxide. FIG. 8B is related with the electrolyzer,and the cell structure is described herein above. It uses nickel mesh asOER catalyst and AEM in the middle. The ORR catalyst used in thisexperiment is Au mesh electropolymerized with cobalt salen). The cellincludes a gas diffusion electrode as the cathode, wherein the gasdiffusion electrode includes a surface layer modified with an Au/cobaltcoordination complex catalyst. More particularly, 5-10 nm Aunanoparticles were deposited on the surface via sputtering. Then, the Aucovered substrate was electropolymerized in a manner analogous to thatdescribed in Example 5. The cathode can also provide humidified O₂ andallows aqueous hydrogen peroxide solution to be removed. The anode canallow oxygen evolution. The reactions at the anode and cathode of thecell are as follows:Anode: 4OH⁻→4e ⁻+2H₂O+O₂Cathode: O₂+4H₂O+2e ⁻→2OH⁻+H₂O₂Potassium permanganate was used to characterize peroxide production.FIG. 8B shows that the peroxide producing current efficiency of thecatalyst can be over 90% in a wide potential range.

Example 8 Reversible Air Battery

A zinc air battery system comprising an Au/Co salen catalyst layer canbe prepared. See FIG. 9A. The battery can comprise a zinc electrode(e.g., a porous zinc electrode) and an air electrode that allowshumidified oxygen to enter the system. The reactions at the zincelectrode and the air electrode are as follows:Zinc electrode: Zn+4OH⁻→Zn(OH)₄ ²⁻+2e ⁻Air electrode: O₂+2H₂O+2e ⁻→H₂O₂+2OH⁻

An anion exchange membrane comprising any desired material as would beapparent to one of ordinary skill in the art upon a review of theinstant disclosure, including but not limited to materials disclosed inG. Merle et al., Journal of Membrane Science 377 (2011) 1-35) can beused to separate the zincate from the air electrode and to provide analkaline working environment. Hydrogen peroxide solution can be producedduring the discharge and preserved out of the cell. The polarizationperformance of a system similar to that of FIG. 9A is shown in FIG. 9B.The electrolyte used in this experiment is 1M KOH+2 mM Cobalt salen+10mM EDTA, rather than the water solution in the graph. In someembodiments, an AEM is employed that allows for the KOH solution not tobe used. In this case, the modified cobalt salen is used on the catalystlayer, and water and small amount of peroxide stabilizer is used aselectrolyte. Assuming that the concentration of peroxide can reach 50weight % to 80 weight % out of the cell at the end of the discharge, theenergy densities for this kind of zinc air battery are calculated to be397 Wh/kg to 489 Wh/kg.

Example 9 Nano-Size Gold-Based Catalyst

The use of nanoscale catalysts opens up a number of possibilities ofimproving catalytic activity and selectivity. Besides the increase insurface area, the catalytic properties of catalyst can also be greatlyinfluenced by nano size. Both the bulk atom to surface atom ratio andthe edge to surface ratio are greatly increased. Since the catalystworking mechanism often is based on the interaction between the catalystand the reactant, the particle edge catalytic activity is different fromparticle surface. Therefore, when these two ratios increase to severalmagnitudes larger than the bulk phase, this will have a great influenceon catalytic activity.

Nano gold particles with size over 8 nm generally have similar catalyticactivity to bulk phase Au. Therefore, a first experiment involved makinga nano Au particle size in the range of 8 nm to 25 nm. One of thesimplest ways to produce Au nanoparticle of this size is use trisodiumcitrate as reducing agent to reduce hydrogen tetrachloroaurate insolution. In this method, the size of the particle is controlled by thereaction time and temperature. After three hours reaction, the particlesize is approximately between 15 nm to 20 nm. The actual particle sizeis measured by XRD experiment, as described in Example 1 herein above.

A nano Au particle was deposited on an electronically conductivesupport, carbon or titanium oxide. Carbon black, Vulcan XC-72, was usedfor the experiment. Since carbon supports provide most of the surfacearea for the electrode and are two electron ORR catalysts, cobalt salensolution was not used to modify the Au on carbon catalyst. Thealternative method employed was to have cobalt salen dissolved in theink to form an electrode.

The ink composition and preparation method are described in Example 1.Depositing the ink onto RDE electrode also follows the previousdescription in Example 1. To verify whether cobalt salen and Au nanoparticle have a strong interaction, the easiest method is a continuousCV in nitrogen purged alkaline solution. A continuous 40 cycles CV justafter this electrode was immersed into the solution was observed. Thetendency showed in this CV indicates the interaction between cobaltsalen and electrode gets weaker with time after immersion into solution.The cobalt salen, which was previously dissolved in the solution,diffuses into the bulk alkaline solution. However, it is believed thatthis interaction should have sufficient influence on the surfaceproperties of Au nano particles to change the ORR catalytic activity ofthe catalyst.

To verify this, a comparison experiment was designed. Using the same Auon C powder, another ink with the same composition except for the cobaltsalen was made. The catalytic activity of these two catalysts was testedunder the same conditions.

In a pure 1M KOH solution, this comparison experiment shows exactly thesame difference with previous results on bulk Au electrode. The catalystmade from ink without cobalt salen shows no sign of reversibility andalmost no sign of peroxide oxidation. On the other hand, the catalystmade from ink with cobalt salen shows very good reversibility andperoxide oxidation. However, the reversibility for Au on a carboncatalyst modified with cobalt salen does not show reversibility similarto the bulk Au electrode. Two possible reasons for this are that (1) thecarbon support is actually producing peroxide but not oxidizing it back,so the overall reversibility of the electrode is not very good and (2)the surface area of nano Au in specific area is larger than for the bulkAu electrode and the peroxide produced is more likely to be furtherreduced. See FIG. 13.

It will be understood that various details of the presently disclosedsubject matter may be changed without departing from the scope of thepresently disclosed subject matter. Furthermore, the foregoingdescription is for the purpose of illustration only, and not for thepurpose of limitation.

What is claimed is:
 1. An electrochemical cell comprising: an air electrode in flow communication with a storage tank containing an aqueous solution of hydrogen peroxide; a metal electrode; a catalyst layer in contact with the air electrode or a gas diffusion layer associated with the air electrode; and a separator layer in contact with the metal electrode and catalyst layer, wherein the catalyst layer includes a catalyst for two electron reversible oxygen reduction, wherein the catalyst comprises (a) gold and (b) a cobalt coordination complex or polymer thereof, and wherein the cobalt coordination complex comprises a cobalt ion chelated by a tetradentate organic chelating ligand.
 2. The electrochemical cell of claim 1, wherein the tetradentate organic chelating ligand is N,N′-bis(salicylidene)ethylenediamine.
 3. The electrochemical cell of claim 1, wherein the cobalt coordination complex has a structure of Formula

wherein each of R₁, R₂, R₃, R₄, R₅, R₆, R₇ and R₈ is independently selected from the group consisting of H, alkyl, cycloalkyl, aralkyl, aryl, heteroaryl, alkoxy, aryloxy, aralkoxy, thioalkyl, thioaralkyl, thioaryl, aminoalkyl, aminoaralkyl, aminoaryl, and a conducting polymer, and wherein R is alkylene or arylene.
 4. The electrochemical cell of claim 1, wherein the cobalt coordination complex has a structure of Formula (I):

wherein each of R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈ is independently selected from the group consisting of H, C₁₋₂₀ alkyl, cycloalkyl having 3 to 10 carbon atoms, C₁₋₂₀ alkoxy, C₁₋₂₀ thioalkyl, C₁₋₂₀ aminoalkyl, and a conducting polymer, and wherein R is C₁₋₂₀ alkylene or arylene.
 5. The electrochemical cell of claim 1, wherein the separator layer is selected from the group consisting of a liquid organic electrolyte, a polymer electrolyte, an aqueous electrolyte, a solid-state electrolyte, and a porous insulative film impregnated with electrolyte solution.
 6. The electrochemical cell of claim 1, wherein the metal electrode is porous.
 7. The electrochemical cell of claim 1, wherein the aqueous solution of hydrogen peroxide comprises a peroxide stabilizer selected from the group consisting of sodium silicate, a transition metal ion complexing agent, and a transition metal chelating agent.
 8. The electrochemical cell of claim 1, wherein the aqueous solution of hydrogen peroxide comprises a peroxide stabilizer selected from the group consisting of ethyllenediamine tetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and a phosphonate.
 9. The electrochemical cell of claim 1, wherein hydrogen peroxide is transferred out of the electrochemical cell for storage, purification, or stabilization.
 10. The electrochemical cell of claim 1, wherein the metal electrode is a lithium electrode. 